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14 4 Stroke Top End Inspection

1. If the contact area appears to be closer to the bottom of the valve seat you should narrow the seat from the top thus helping to center the contact area Likewise if the contact area is closer to the top of the valve seat you can center the contact area by narrowing the seat from the bottom Examples of this are shown in Figure 28 The blue dye should have transferred evenly from the face to the seat If the valve seat is properly machined and isn t out of round the dye should be visible all the way around the valve seat If the dye is distributed unevenly either the valve face or the valve seat is out of round and should be refinished You should repeat this procedure until the seat is narrow enough and the contact area is closely centered on the seat s width Before you recheck the seal though be sure to wipe off any dye remaining from the previous check Then apply new dye to the valve and repeat the procedure When the valve seat has been cut to its proper size and shape the valve and seat are ready for the valve lapping process Four Stroke Engine Top End Inspection 45 FIGURE 28 Determining Where to Narrow the CONTACT TOO HIGH Valve Seat Width Copyright OLD SEAT by American Honda Motor Co Inc WIDTH and reprinted with permission 32 CONTACT TOO LOW OLD SEAT Valve Lapping Lapping is the process of mating the valve and the seat together to ensure a tight fit Valve lapping produces the closest possible fit
2. Next we explain how to visually inspect and measure the components of the engine top end Throughout the discussion we point out the tools used in the process and provide you with some review information about the function of certain engine components In addition to the disassembly process we also cover reassembly procedures When you complete this study unit you ll be able to e State the procedures for removing a four stroke engine from the chassis e State the procedures for disassembling the top end of a four stroke motorcycle or ATV e Diagnose common problems related to four stroke engine top end failure e Inspect the components of the top end of a four stroke motorcycle or ATV e State the procedures for reassembling the top end of a four stroke motorcycle or ATV Contents INTRODUCTION a ses alg ae A oe as BAEK ee eee eS 1 Repair Procedures Engine Disassembly and Assembly FOUR STROKE ENGINE REMOVAL 2 Cleaning the Engine and Draining the Fluids Removing the Engine Body Components Removing the Engine DISASSEMBLING THE FOUR STROKE ENGINE TOP END ASSEMBLY 10 Removing the Spark Plug Removing the Cylinder Head Cover Removing the Camshaft Removing the Cylinder Head Disassembling the Cylinder Head Removing the Cylinder Removing the Piston and Piston Rings FOUR STROKE ENGINE TOP END INSPECTION 19 Inspecting the Piston Inspecting the Piston Rings Measuring the Piston Rings Inspe
3. cutaways friction heat The skirt 10 eo NS EN Ua L A DUTEN POUTRE ES S Boring it to a larger diameter Cylinder bore gauge taper Detonation preignition False machinist ruler Vernier caliper lapping Freeze the valve guide to shrink it and heat the cylinder head to expand it Piston crown oil control narrow pitch cylinder head Exhaust Room temperature 2 1 The crankshaft turns two times for every turn of the camshaft Examination ONLINE EXAMINATION For the online exam you must use this EXAMINATION NUMBER 03301100 When you re confident that you ve mastered the material in your studies you can complete your examination online Follow these instructions 1 Write down the eight digit examination number shown in the box above 2 Click the Back button on your browser 3 Click the Take an Exam button near the top of the screen 4 Type in the eight digit examination number
4. inspect the condition of the rocker arm s sliding surface for pitting and galling If damage or wear is noted the affected part must be replaced ROCKER ROCKER ARM SLIDING SHAFTS SURFACES Copyright by American Honda Motor Co Inc and reprinted with permission Four Stroke Engine Top End Inspection 13 Removing the Camshaft Before the camshaft can be removed you must relieve the tension on the cam chain After removing the camshaft take special care not to drop the cam chain into the crankcase as it may be very difficult to retrieve Use a screwdriver or a piece of wire to keep the cam chain in sight Note The illustrations used in the following example procedure are courtesy of the American Suzuki Motor Corporation 1 Remove the cam chain tensioner 2 Remove the tensioner bolt to CAMSHAFT CAMSHAFT relieve tension on the cam drive SPROCKET SPROCKET chain BOLT 3 Rotate the engine and remove all fasteners attaching the sprocket to the cam The engine rotates easily if you ve already removed the spark plug as was suggested previously CAM CHAIN Note When working on a DOHC engine the procedures are the same with one exception On a DOHC there are two camshafts held in place by cam caps In most cases you ll not be required to remove the camshaft sprockets on a DOHC top end to remove the cams CAM CHAIN TENSIONER BOLT 14 Four Stroke Engine Top End Inspection Removing the Cyli
5. s specification If the cylinder head s surface is within specifications then the head mating surface area should be satisfactory However if you find that the cylinder head is warped more than is allowed the head will need to be either resurfaced by a machine shop or replaced In Four Stroke Engine Top End Inspection 33 most cases cylinder heads used on motorcycle and ATV engines are quite expensive Therefore it s generally more cost effective to repair a head Inspecting the Valve Assembly The valves in a four stroke motorcycle or ATV engine perform two vital functions during engine operation Figure 13 The intake valve allows the air and fuel mixture to enter the combustion chamber so the mixture can be burned to produce power The exhaust valve al lows exhaust gases to exit the combustion chamber so a fresh supply of air and fuel can enter The valves must seal tightly when they re closed so the mixture in the cylinder doesn t escape during the compression and power strokes Therefore for an engine to operate properly its valves must be in good condition FIGURE 13 Intake and Exhaust valves at Work INTAKE EXHAUST INTAKE EXHAUST VALVE VALVE VALVE VALVE OPEN CLOSED CLOSED OPEN EXHAUST STROKE INTAKE STROKE EXHAUST STROKE The parts of a typical valve are labeled in Figure 14 The valve areas you ll need to check during a rebuild are shown on the right side of this figure e Dimension A is the diamete
6. Disassembling the Cylinder Head After you ve completed your visual inspection of the cylinder head you can disassemble its parts You ll need a valve spring compressor for the removal and replacement of the valves This tool is used to com press the valve springs while also preventing the valves from open ing In this way the valve keepers can be removed and the valve components disassembled 16 Four Stroke Engine Top End Inspection 1 Position the valve spring compressor so that one end of the compressor is on the valve head and the other is on the valve collar 2 Tighten the valve spring compressor to compress the valve spring 3 With the spring fully compressed remove the valve keepers using a pair of needle nose pliers or a small magnet 4 Slowly and carefully release the pressure of the valve spring VALVE SPRING compressor so you can extract the COMPRESSOR KEEPER valve from the guide Courtesy of American Suzuki Motor Corporation Removing the Cylinder It s necessary to remove the entire cylinder to expose the piston and rings Be careful to keep the internal parts of the engine clean and free from foreign objects Note The illustrations used in the following example procedure are courtesy of the American Suzuki Motor Corporation 1 If the cylinder is secured to the crankcase with cylinder base holding nuts as shown in the figure to the right they must be unscrewed before the cylinder can be lifte
7. Inspection proper measuring instruments and record all your measurements ac curately When you disassemble and rebuild an engine examine the condition of each component Check the parts before you clean them For the most part this should be a preliminary examination The as is condition of the parts can reveal a lot about the operation of an engine After you ve examined the parts and recorded your observations clean the parts thoroughly and proceed with the rebuild Then measure and de termine which parts need to be replaced and which can be reused Let s begin our demonstration of the four stroke engine top end in spection by removing the engine from the chassis FOUR STROKE ENGINE REMOVAL Many four stroke motorcycle and ATV engines use long stud bolts to secure the head and cylinder to the engine cases Stud bolts have threads at each end One end is screwed into the engine crankcase a nut is screwed on the other end outside the cylinder head This holds the head to the cylinder Removal and replacement of stud bolts is very difficult and requires more time than removing and replacing the engine in the frame Therefore these engines are best removed be fore attempting cylinder repair cylinder head repair or any repair re quiring major engine components to be disassembled Procedures to remove two stroke engines from the frame were described in the last study unit Procedures for the removal of a four stroke motorcycle or
8. a tough material to withstand extreme heat In most engines valve seats are made of a very hard steel alloy In an aluminum cylinder head valve seats are usually in the form of inserts pressed into place by the manufacturer Because of the extreme heat under which the valve seats operate the seats like the valves become distorted and worn out over time When the valve face or the valve seat becomes distorted the sealing surfaces no longer match up therefore the valve doesn t seal completely when it s closed For this reason valve seats normally need to be refinished during the rebuilding process The refinishing process called valve seat refacing restores the valve seat to a perfectly round shape with a smooth sealing surface Also the valve seat is beveled to match the angle of the valve face Figure 22 By refinishing the valve seat you ensure that the valve forms a proper seal when closed FIGURE 22 The Various Angles of a Valve and Its Seat Courtesy of American Suzuki Motor Corporation VALVE SEAT The most common hand tools used to refinish a valve seat are pic tured in Figure 23 The appropriate angle for a valve seat is found in the service manual for the motorcycle or ATV engine The equipment for valve seat grinding is normally available from specialty tool manufacturers or from automotive supply stores The valve reconditioning tool uses a pilot to position the cutting device and to ensure that the tool rem
9. at the three different locations in the figure The middle bevel 45 is the actual seat that makes contact with the valve face The width of the valve seat must now be measured and checked against specifications A machinist s ruler or a Vernier caliper can be used to measure this dimension Making the seat too narrow prevents the valve from sealing properly Check the manufacturer s manual for the exact specifications for your engine FIGURE 25 The finished valve seat should have VALVE SEAT at least three cut angles 60 45 15 After cutting the seat and measuring its width you may find the valve seat is too wide The width can be decreased by partially recutting the seat with either a wider or narrower cutting device Most valve seats are usually beveled to 45 degrees By using a cutting device designed to make a cut at a smaller angle to the horizon normally 15 to 32 de grees the top edge of the valve seat is ground down thus decreasing the overall width of the seat Figure 26 By using a cutting device de signed to make a cut at a larger angle to the horizon normally 60 to 75 degrees the lower edge of the valve seat is ground down Figure 26 Either of these methods can be used to narrow a valve seat to its proper width foo GRINDING TOP EDGE OF SEAT GRINDING BOTTOM EDGE OF SEAT When the seat is narrowed to its proper width the seat should be fin ished by cutting it lightly with the 45 degree cutter Th
10. between the valve face and the valve seat As a general rule valves should be lapped to the seats any time the valves have been removed from the engine even if they appear to be in good shape and you don t plan on reconditioning them Even brand new valves should be lapped before installing them into an engine Valves are lapped using a grinding paste or lapping compound a sub stance that feels a lot like ordinary toothpaste but contains fine abra sive grains When the compound is rubbed onto metal the abrasive grains smooth the metal s surface The paste is used with all four stroke engines Lapping compound is a common product that can usually be purchased from a local auto parts store The compound is available in versions with grains of varying abrasives Usually a coarse grain compound is used when first starting to lap the valves the final lapping is done using a fine grain compound Organizing the valves Before beginning the lapping process organize the valves you ll be installing in the engine You may want to temporarily insert all the valves into their guides to keep them organized Then remove them one at a time and lap each one to its own valve seat Another way to keep the valves organized is to cut several holes into a piece of cardboard The holes should be slightly larger than the valve stem diameter Then use the cardboard as a holder for the valves Write the location of each valve on the cardboard 46 FI
11. control rings are in three separate pieces that in clude two side rails and the oil ring itself The second ring to be in stalled is the scraper ring It s generally a dark colored ring The top ring is a compression ring and most likely has a chrome plated surface area on the outside of the ring itself Note The illustrations used in the following example procedure are courtesy of the American Honda Motor Corporation and reprinted with permission 1 Install the piston rings by carefully spreading the ring gap to increase the ring size Slip the opposite side of the ring over the piston into the appropriate piston groove Install the rings so their end gaps are 120 apart from each other 2 Place a clean rag over the base of the cylinder to prevent any foreign objects from falling into the crankcase SIDE RAILS MARK COMPRESSION RING SCRAPER RING OIL RING KD Ee ZZZ SSS SPACER GAP 20 mm 0 8 in 20 mm 0 8 in OR MORE OR MORE 50 3 Apply a small amount of oil or moly lube to lubricate the piston wrist pin and the small end of the connecting rod 4 Install the piston pin through the piston making sure that the mark on the piston is facing in the correct direction 5 Install new piston pin retaining rings using a pair of needle nose pliers Four Stroke Engine Top End Inspection PISTON WRIST PIN CONNECTING ROD PISTON PIN MARK PISTON PIN RETAINING RING
12. diameter of the guide is quite small a small bore gauge is required to measure that dimension Figure 18 As discussed earlier a typical outside micrometer can be used to measure the valve stem s diameter The stem diameter is then subtracted from the guide diameter to find the clearance between the stem and guide Finally the calculated clearance is compared with the manufacturer s specifications given in the service manual FIGURE 18 Using a Small Bore Gauge to Measure a Valve Guide Copyright by American Honda Motor Co Inc and reprinted with permission 38 Four Stroke Engine Top End Inspection Replacing Valve Guides Let s look closely at the construction of a valve guide A valve guide is located in the cylinder head Figure 19 Generally valve guides are made of soft metals such as bronze or cast iron to reduce the amount of friction created by the moving valve stem FIGURE 19 Valve guide location is shown in this cutaway illustration Copyright by American Honda Motor Co Inc and reprinted with permission Worn valve guides can be removed with a driver and a ball peen hammer or a press such as an arbor press Whether you select a hammer or a press you should use a special driver tool when remov ing the old guide Figure 20 These tools are available from the motor cycle and ATV manufacturer When driving out the old valve guide be sure that the cylinder head is supported so it won t mov
13. in this study unit are general in nature and not in tended to be used for actual disassembly and repair Their purpose is to familiarize you with the types of activities you ll encounter Al ways refer to the appropriate motorcycle or ATV service guide for maintenance information The service guide contains all the infor mation to do the job correctly including detailed instructions for the specific make and model of motorcycle or ATV special tools and service tips Above all the service guide contains the appropriate safety information Engine Disassembly and Assembly When disassembling an engine be careful not to remove parts that don t need to be removed Close inspection of the motorcycle or ref erence to a service manual can save many hours of time spent remov ing and replacing parts unnecessarily When rebuilding the top end components of any engine you need to be both patient and precise Before you begin any repair job take time to assemble the proper tools and the materials you ll need mi crometers valve refacing tools lapping tool and compound cleaning solvent and so on Also be sure to have the proper service manual for the motorcycle or ATV engine on which you ll be working If an engine is to work properly the measurements involving its piston and valves must fall within the manufacturer s specifications Be pa tient and careful as you make the required measurements Use the 2 Four Stroke Engine Top End
14. inside the engine to burn improperly True or False All four stroke engine valves must be refaced before installing them into the cylinder head The valve seat can be measured using a ora The process used to mate the valve and the seat together is called What can be done to make the installation of a valve guide in a cylinder head easier Check your answers with those on page 63 FOUR STROKE TOP END REASSEMBLY We ve discussed removal of the engine from the chassis disassembly procedures for the four stroke engine top end and inspection and measuring procedures for the various top end components Now it s time to begin the reassembly procedures Note that we won t explain replacing the engine back into the chassis but will cover this subject in the next study unit Because the top end was disassembled right down to the piston we must begin our reassembly at that point We ll again describe each procedure in steps as we did when we disassem bled the top end Four Stroke Engine Top End Inspection 49 Installing the Piston Rings and Piston It s generally easier to install the piston rings on the piston while it s removed from the connecting rod Piston rings have markings on them indicating which side of the ring goes towards the top of the piston crown Four stroke engines will as mentioned earlier have three piston rings The bottom ring is for oil control and should be installed first Most modern day oil
15. of the fuel mixture burning improperly in the cylinder If the fuel mix ture ignites incorrectly a violent explosion can result The concen trated heat created in such an explosion can burn a hole right through the piston crown Also the explosion itself can be powerful enough to break right through the top of the piston The following two terms de scribe different conditions that cause the fuel mixture to burn improp erly Preignition When preignition occurs the air and fuel mixture ignites in the combustion chamber before the spark plug actually fires This may sound strange How can the mixture ignite before the plug fires The explanation is based on the fact that the burning air and fuel mixture produces a lot of heat The lingering high temperature in the combustion chamber can cause the mixture to ignite without a spark Preignition can also be caused by excessive compression of the air and fuel mixture The carbon that sometimes builds up inside the cylinder reduces the overall volume of the combustion chamber The chamber s reduced volume results in an increase in the compression force exerted on the air and fuel mixture Detonation In the engine condition known as detonation the air and fuel mixture fails to burn smoothly Instead the mixture begins to burn in only one area of the combustion chamber Then as the pressure and heat in the chamber increase the mixture ignites a Four Stroke Engine Top End Inspection second
16. on to the valve guides These components of the valve as sembly are used to properly position the valves in the engine and to guide the valves as they move up and down The valve guide s job is difficult Not only must it keep the valve in position but it must also allow the valve to move freely up and down even in extreme heat The extreme heat around the valve guide makes it difficult for oil to properly lubricate the guide If insufficient oil is available for lubrica tion excessive friction rapidly wears down the guide If more oil is present than is needed the excess oil becomes baked onto the valve stem The baked on oil can build up and block the valve openings Valve guide wear This is a phrase referring to the amount of clearance between the valve stem and the valve guide Keep in mind that the valve opens and closes thousands of times each minute This leads to wear in the valve guide As the valve guide wears the valve begins to move slightly side to side as it opens and closes This side to side movement if excessive can cause the valve to seat improperly and thus fail to completely seal the cylinder For this reason the guides must be checked and replaced or repaired if they re found to be worn beyond the manufacturer s specifications Determining valve guide wear Valve guide wear is determined by comparing the measurement of the inside diameter of the guide to the outside diameter of the valve stem Because the inside
17. time in another area of the combustion chamber Thus two separate flames can burn at the same time in the chamber When the two flames collide a shock wave is created The shock wave effectively hammers the top of the piston and the piston rings Eventually this hammering or detonating damages the piston and rings You may already be familiar with engine detonation Most of today s automobiles exhibit it under certain conditions particularly when the vehicle is under a load for example going uphill The pinging noises that are often heard in such a situation indicate that detonation is oc curring inside the engine s cylinders The most common cause of detonation is the use of gasoline with an octane rating that s too low for the engine The recommended octane rating for the fuel in a particular engine can be found in the owner s manual and in the manufacturer s service manual for that engine Detonation may also be caused by incorrect ignition timing If the ig nition timing is too advanced the spark occurs earlier than it should In this case the fuel mixture ignites and starts to burn when the piston is still rising on the compression stroke This disruption of the normal burning pattern results in detonation Inspecting the Piston Rings The next step is to check the piston rings for signs of damage The condition of the old piston rings can provide clues to certain engine problems For example small scratches found on
18. ADJUSTING SCREW ROCKER ARM Four Stroke Engine Top End Inspection ROCKER ARM FEELER GAUGE VALVE STEM CAM LOBE FEELER GAUGE Four Stroke Engine Top End Inspection 61 This completes the top end repair and inspection of the four stroke engine If the engine you re working on doesn t require the engine to be removed from the frame consult your service manual to determine the order of reassembly of the components to complete your repairs As mentioned earlier we ll install the engine back into the chassis in the next study unit after we disassemble inspect and reassemble the lower end of the engine Road Test 4 1 Which side should the marking of the piston rings face 2 The piston ring that may be in three separate pieces is called the ring 3 The valve springs must be installed so the of the spring sits on the surface of the head 4 A mark on the camshaft sprocket is usually aligned with a timing mark on the 5 Which valve usually has more clearance between it and the rocker arm 6 At what engine temperature are the valves adjusted 7 What is the gear ratio between the crankshaft and the camshaft Check your answers with those on page 63 Road Test Answers False clean With the drive chain still installed put the engine in gear and hold the rear brake Radiator cap three To help remove the excess oil from the cylinder walls crisscross Valve spring compressor
19. ATV engine are as you ll see very similar Removal of most motorcycle engines from the frame follows a pat tern that is certain parts must be removed on each model This pat tern generally applies to all makes and models of motorcycles and ATVs Parts that need to be removed usually include but aren t lim ited to the following e Battery when applicable This is a safety measure to prevent the possibility of fire The spark of an accidentally grounded positive battery cable on spilled gasoline may start a fire e Fuel tank This lets you free control cables and engine mount bolts e Fairings and bodywork e Chain e Control cables Four Stroke Engine Top End Inspection 3 e Ignition and other electrical wiring that may be attached to the engine e Engine mounting bolts Certain models require that some smaller parts be dismantled to fa cilitate removal of the engine It s not possible to list each part for each brand and model You need to refer to your service manual for the exact information concerning engine removal However you need to remove only those components that interfere with lifting the engine in or out In the following section we describe procedures for removing the en gine from our example e Motorcycle a liquid cooled double overhead cam DOHC Kawasaki GPZ 500S shown in Figure 1 e ATV an air cooled single overhead cam SOHC Suzuki LT 250S shown in Figure 2 We discuss the specific diffe
20. Courtesy of American Suzuki Motor Corporation Installing the Cylinder Head Place the cylinder head onto the cylinder and secure it by torquing the four outside bolts as well as the cylinder head base fasteners Remember to tighten the fasteners in a diagonal or crisscross pattern After properly tightening the cylinder head base fasteners re torque the cylinder mounting fasteners to the proper specification 54 Four Stroke Engine Top End Inspection Installing the Camshaft The overhead camshaft engine is the most widely used engine in the motorcycle and ATV industry We ll focus on this type of engine as we discuss the installation of the camshaft Following our general dis cussion of camshaft installation you ll find procedures for both the single overhead cam and double overhead cam designs You should now be aware that all four stroke engines must be ad justed or timed so each rotating part is in the proper position at the proper time in relation to the other moving engine parts The timing of the overhead camshaft rotation in relation to the crankshaft rota tion is vital This is because the camshaft must open the valves and allow them to close at specific degrees of rotation of the crankshaft The induction and exhaust of gases must take place at specific times in the engine cycle these times are controlled by the camshaft Relationship of Camshaft and Crankshaft Rotation The timing for the valves to open is determined
21. GURE 29 Procedure for Proper Valve Lapping Copyright by American Honda Motor Co Inc and reprinted with permission Four Stroke Engine Top End Inspection Lapping the valves To begin lapping the valves apply a thin coating of lapping compound to the face of a valve When you ve covered the contact area insert the valve into the valve guide and push it down until it makes contact with the valve seat When installed each valve rotates rapidly within its own seat Remember that the abrasive lapping compound is between the valve face and the valve seat Therefore when the valve is rotated the abrasives in the compound wear away the surfaces slightly thereby mating them to one another A lapping stick is a tool used to rotate the valves The lapping stick consists simply of a round wooden or plastic shaft with a suction cup on the end The suction cup is attached to the head of the valve To help the suction cup stick better many technicians moisten the cup slightly before attaching it After you ve attached the lapping stick you can rotate the valve back and forth by spinning the shaft of the tool between the palms of your hands Figure 29 While rolling the shaft back and forth apply a moderate amount of downward pres sure This helps the lapping compound to mate the valve and the seat together MOVE THE VALVE UP AND DOWN WHILE ROTATING THE VALVE ROTATE THE VALVE LAPPING AND PRESS LIGHTLY COMPOUND To check the v
22. REMOVER OR Motor Co Inc and reprinted with DRIVER permission Replacing the valve guide When the guide bore is determined to be in good condition you re ready to install the new guide The new valve guide must fit very tightly in the guide bore Remember that when metals are chilled they contract get smaller and when heated they expand get larger Therefore to make it easier to insert the tightly fitted valve guide place the guide in a freezer for about an hour The cold temperature causes the valve guide to shrink Also you may want to heat the cylinder head on a hot plate to allow the head to expand When the guide has cooled off and the head has heated up insert the guide into the cylinder head A special driver with a ball peen hammer or arbor press can again be used to install the new guide into the cylinder head Because the cold guide has shrunk somewhat and the cylinder head has been heated the guide should slide into the guide bore easily When you insert the guide be sure that it s placed in its proper position as described in the engine s service manual Reaming the valve guide After you ve replaced the guides you ll need to ream out the newly inserted valve guides to meet specifications A new guide has a slightly smaller inside diameter than is necessary After the guide is installed its inside diameter must be enlarged to be slightly larger than the valve stem The hole in the guide is enlarged using a r
23. SHOP TOWEL CUTOUT Four Stroke Engine Top End Inspection 51 Installing the Cylinder The procedure for rejoining the cylinder to the crankcase in the four stroke engine is much the same as that for the two stroke engine Note The illustrations used in the following example procedure are courtesy of the American Suzuki Motor Corporation unless other wise noted 1 Clean the old gasket material from all surfaces PISTON RING COMPRESSOR 2 Replace the cylinder base gasket making sure it s properly aligned by installing any alignment or dowel pins 3 When installing a multicylinder engine lightly oil and compress the piston rings with a ring compressor In fact lightly oil all moving parts as well as the cylinder bore as you assemble them When assembling a single cylinder engine you may prefer to compress the rings with PISTON BASE your fingers Copyright by American Honda Motor Co Inc and reprinted with permission 4 Lower the cylinder over the piston then remove the ring compressors if they re being used Make sure that you pull the cam chain out of the crankcase cavity to prevent the chain from being caught between the cylinder and the crankcase Push the cylinder down tightly onto the crankcase CAMSHAFT DRIVE CHAIN 5 Secure the cylinder base nuts if base nuts are used Tighten the base nuts to the manufacturer s suggested torque 6 Install the cam chain tensioner guide into the cylind
24. SPECTION There are many components in the four stroke top end that require careful inspection and measurement We ll begin our visual inspec tion of a four stroke engine with the piston Then we ll move on to the piston rings wrist pin cylinder head valve train and camshaft Inspecting the Piston As you ll recall the piston is the cylinder shaped component that moves up and down the cylinder bore The piston assembly consists of the piston itself its wrist pin or piston pin and the piston rings As you re now aware an engine produces its power by burning the air and fuel mixture in the combustion chamber directly above the piston Each time the spark plug fires the air and fuel mixture ignites with an explosive force The burning process heats the gases causing them to expand rapidly forcing the piston down the bore The piston movement is what allows the engine to perform useful work As you can imagine the piston has to withstand a tremendous amount of physical force as well as extremely high temperatures during engine operation Therefore as part of the rebuild procedure you must care fully inspect the entire piston assembly for damage 20 FIGURE 3 Problems to Look for When Inspecting a Piston Image courtesy of Yamaha Motor Corporation U S A Four Stroke Engine Top End Inspection Checking the Piston for Damage Start your inspection of the assembly with a visual examination of the piston itself Fi
25. Study Unit Four Stroke Engine Top End Inspection Ed Abdo Author Acknowledgment Edward Abdo has been actively involved in the motorcycle and ATV industry for more than 25 years He received factory training from Honda Kawasaki Suzuki and Yamaha training schools He has worked as a motorcycle technician service manager and Service Parts department director After being a chief instructor for several years Ed is now the Curriculum Development Manager for the Motorcycle Mechanics Institute in Phoenix Arizona He is also a contract instructor and administrator for American Honda s Motorcycle Service Education Department Preview Top end engine disassembly is a process by which all of the components of the top end of an engine are removed Engine disassembly is a task you ll perform often in your job An engine may be disassembled to make needed repairs or to complete the first step in an engine rebuild During an engine rebuild the engine is completely cleaned and restored to a like new condition This study unit introduces you to the procedures used to disassemble the four stroke engine top end assembly You ll start by learning the important preliminary steps you must complete before taking the engine apart Then you ll learn the engine disassembly procedures The procedures we describe apply to almost all four stroke engines regardless of model or manufacturer however we illustrate the procedures with real engine examples
26. This ensures the engine is at TDC compression for that particular cylinder as the intake and exhaust valves will both be closed in this position Engage the chain onto the camshaft sprockets while you ensure the marks on the camshaft align in parallel with the surface of the cylinder head Some DOHC engines verify that the cam chain is installed correctly by counting the number of cam chain pins between the marks in the cam shafts Four Stroke Engine Top End Inspection LOCATING PIN HOLE CAMSHAFT MARKS Courtesy of American Suzuki Motor Corporation FRONT TIMING CAM CHAIN PIN TIMING MARK EXHAUST CYLINDER HEAD UPPER SURFACE Courtesy of American Suzuki Motor Corporation Four Stroke Engine Top End Inspection 57 Installing the Chain Tensioner 1 Compress the cam chain tensioner before installing it into the engine 2 Mount the cam chain tensioner back onto the cylinder RACHET MECHANISM PUSH ROD Courtesy of American Suzuki Motor Corporation 3 Install the tensioner spring into the tensioner assembly and A NA SPRING tighten the spring holder bolt to on HOLDER the specified torque F l BOLT Courtesy of American Suzuki Motor Corporation Installing the Cylinder Head Cover SOHC Designs After the camshafts and valve assembly have been properly installed and timed to the crankshaft it s time to check for the proper valve clearance SOHC engines require the
27. U S A 10 Four Stroke Engine Top End Inspection Road Test 1 At the end of each section of Four Stroke Engine Top End Inspection you ll be asked to check your understanding of what you ve just read by completing a Road Test Writing the answers to these questions will help you review what you ve learned so far Please complete Road Test 1 now 1 True or False When you disassemble the top end assembly of motorcycles and ATVs the engine is normally left in the chassis 2 Before doing any work on a motorcycle or ATV you should always thoroughly the machine 3 How can you make removal of the engine sprocket bolt easier as you prepare to remove the engine from the chassis 4 Ona liquid cooled engine what should you remove besides the drain bolt to allow the coolant to drain out the engine easier Check your answers with those on page 63 DISASSEMBLING THE FOUR STROKE ENGINE TOP END ASSEMBLY After you ve removed all parts that might interfere with disassembling the top end assembly or removed the engine from the chassis as de scribed in the previous section of this study unit you re ready to start disassembling the engine Our example for illustrative purposes is the single cylinder Suzuki LT 250S ATV which uses a single overhead camshaft design We ll explain the different procedures used when disassembling a double overhead camshaft model as needed How ever note that most procedures are identical w
28. ains centered properly The pilot is simply a round piece of metal that fits tightly into the valve guide The cutting device has a hole in its center that fits over the end of the pilot The pilot can thus hold the tool centered in the valve guide Because a pilot must be inserted into the valve guide to refinish the seat the valve guide must be in proper condition before the seat is refinished 42 FIGURE 23 Valve Seat Refinishing Tools Four Stroke Engine Top End Inspection After the pilot is inserted into the valve guide the cutting device is placed over the pilot and into contact with the valve seat The grind ing tool is then rotated as shown in Figure 24 The rotation of the tool removes metal from the valve seat and refinishes its surface Metal is removed from the valve seat until a smooth uniform surface appears At first the tool cuts only in a few spots due to the distortion of the valve seat As the tool continues rotating metal continues to be re moved until the tool cuts evenly at all points all the way around the valve seat When this occurs the seat is completely refinished T HANDLE CUTTERS FIGURE 24 Placement of a Valve Seat Cutting Tool Courtesy of American Suzuki Motor Corporation Four Stroke Engine Top End Inspection 43 FIGURE 26 Using a Cutter to Decrease the Overall Width of the Seat Your finished valve seat should look like the one shown in Figure 25 Note the different angles
29. alve seating remove the valve from the engine and clean away all the lapping compound using solvent and a clean cloth When the valve is clean apply a thin coat of blue dye to the valve face Then insert the valve back into the valve guide Apply a slight downward pressure with your thumb and rotate the valve slightly Remove the valve and observe the valve seat If the blue dye is evenly distributed around the seat the valve has been properly lapped If the dye is distributed unevenly around the seat more lap ping compound should be applied and the valve should be re lapped After all valves have been appropriately lapped the valves and their seats should be thoroughly cleaned with solvent and then with soap and water This removes any leftover lapping compound Remember that lapping compound is abrasive if it s allowed to get into the Four Stroke Engine Top End Inspection 47 working engine it may do serious harm to the bearings and other vital engine parts Inspecting the Camshaft The camshaft is the component that controls the opening and closing of the valves in a four stroke engine As the camshaft spins the cam lobes move the tappets which in turn open the valves Other compo nents to note are the bearing support areas on each end of the camshaft and the large sprocket or gear with which the crankshaft can rotate the camshaft via a chain belt or gears Figure 30 FIGURE 30 The camshaft controls the BEARING open
30. ar to be damaged repair them by running a thread tap through the hole or by installing a new threaded insert Checking Cylinder Head Flatness Now you can move on to check the flatness of the surface where the new head gasket will be installed Remember that the cylinder head must seal tightly to the top of the cylinder The gasket between the head and the engine block can compensate for some variation in flat ness but the surface of the head must still be quite flat or the seal will fail The manufacturer s service manual tells you the maximum amount that the surface of a usable cylinder head can be warped There are two methods you can use to check the flatness of the cylin der head s surface Both methods involve the use of feeler gauges to measure warping Four Stroke Engine Top End Inspection With the first method the surface is checked using a straightedge such as a metal ruler Place the cylinder head so the surface on which the gasket will be installed is facing up To check the flatness place the straightedge across the surface of the head as shown in Figure 12 If you notice clearance anywhere between the straightedge and the head insert the blades of a feeler gauge to measure the warp ing at that point The thickness of the blade that fits the clearance is the amount of warp at that particular location Because the straight edge is narrow it should be moved about the surface of the head to check several locations M
31. by the position of the camshaft This position is indicated by degrees of crankshaft rotation Recall that there are 360 degrees in a circle A quarter of a turn is therefore equivalent to 90 degrees half a turn is equivalent to 180 degrees a three quarter turn is equivalent to 270 degrees The valves must open and close at specified degrees of crankshaft rotation The gear reduction ratio between the crankshaft and the camshaft is al ways 2 1 on a four stroke motorcycle or ATV engine This means that the crankshaft makes two revolutions for each revolution of the camshaft Another way of looking at this relationship is that if the crankshaft is rotated 90 or one fourth of its circle rotation the camshaft must rotate 45 one eighth of this circle of rotation This relationship must remain constant during a complete rotation of the shafts the shafts must always return to their starting points simultaneously Two parts must be timed for their positions in relation to the crank shaft These two parts are the camshaft and the ignition system All timing of the camshaft s position in relation to the crankshaft s posi tion is done by aligning certain marks However there are various ways and various places where these marks may appear One of the principal pieces of information you must have for every four stroke engine is the exact location of the timing marks These marks might be letters dots dashes or simple lines Normally
32. cting the Piston Ring Grooves Measuring the Piston Measuring the Cylinder Measuring the Piston to Cylinder Clearance Inspecting the Wrist Pin Inspecting the Cylinder Head Inspecting the Valve Assembly Inspecting the Camshaft FOUR STROKE TOP END REASSEMBLY 48 Installing the Piston Rings and Piston Installing the Cylinder Installing the Valves Installing the Cylinder Head Installing the Camshaft Installing the Chain Tensioner Installing the Cylinder Head Cover SOHC Designs Checking and Adjusting Valve Clearance vi Contents ROADTESTANSWERS eina Sew e a ee ees EXAMINATION Four Stroke Engine Top End Inspection INTRODUCTION When we speak of repairs to the four stroke engine s top end assembly we re referring to work done on the rocker arm valve pistons rings cylinder camshaft and related parts In this study unit the focus is on disassembly and repair of valves and their components as they pertain to the four stroke motorcycle and ATV engine This is the ma jor physical difference between four stroke and two stroke engines Repairs to the top end assembly of most four stroke motorcycle and ATV engines require that the engine be removed from the chassis Some models however don t require the engine to be removed to re pair parts located above the cylinder The manual for the particular model you re working with tells you which is the case Repair Procedures The procedures
33. cylinder bore wears more on the front and back than on the sides By front and back we mean at a 90 angle to the wrist pin Cylinder measurements are taken from front to back and side to side These areas are called the X and Y axis of measurement To measure the cylinder a cylinder bore gauge is used Figure 9 FIGURE 9 A Cylinder Bore Gauge Copyright by American Honda Motor Co Inc CYLINDER BORE GAUGE and reprinted with permission 28 Four Stroke Engine Top End Inspection Insert the cylinder bore gauge at a point near the top of the cylinder and take a reading of the micrometer The gauge is then moved to a point near the center of the cylinder and a reading is taken there as well Finally the gauge is positioned at the bottom of the cylinder and another reading is taken This is done for both the X and Y axis meas urements to determine the cylinder s trueness Figure 10 The read ings are compared and the difference indicates the amount of wear FIGURE 10 The X and Y Axis Measurements for a Four Stroke Cylinder Copyright by American Honda Motor Co Inc and reprinted with permission MIDDLE The difference between measurements taken on the same axis is known as cylinder taper The difference between the two axes is called out of round The taper and out of round must not exceed factory specifications for the engine on which you re working Each model has its own specifications If the measurement
34. d You may also be required to remove the camshaft chain tensioner chain guide or both CAM CHAIN GUIDE CYLINDER CYLINDER BASE NUTS CRANKCASE Four Stroke Engine Top End Inspection 17 2 Lift the cylinder off the base of the engine to expose the crankcase cavity and the piston along with the crankshaft connecting rod A 3 Place a clean rag in this cavity to prevent foreign particles for example old gasket material piston pin retaining rings broken rings and dirt from entering The rag also protects the piston from damage Removing the Piston and Piston Rings After you ve exposed the piston and rings you ll notice major differ ences between the four stroke piston and the two stroke piston One difference is the use of at least one more piston ring The four stroke engine normally uses one oil scraper ring and two compression rings whereas a two stroke engine typically uses only one or two compres sion rings Why the extra piston ring Remember that oil is mixed with fuel and both are burned in the two stroke cylinder In the four stroke cylinder fuel is burned without oil lubrication is supplied by a separate oil supply An oil scraper ring is used to prevent the oil in the lower end assembly of the four stroke engine from passing the piston and entering the combustion chamber Another difference between the four stroke and two stroke engine is in the top of the pistons Cutaways are often provided fo
35. d piston after the engine cools down again however both the piston and the cylinder wall will probably be badly scored and damaged If the piston clearance is too large the piston isn t held in place and tends to rock back and forth while the engine is running This rocking motion creates a knocking noise and may eventually break the piston skirt In addition the ability of the piston rings to seal the combustion chamber is greatly reduced Determining Piston Clearance To determine the piston clearance in an engine you ll need to meas ure the diameter of both the piston and the cylinder bore Compare your measurements to the manufacturer s specifications Then sub tract the outside diameter of the piston from the inside diameter of the cylinder bore The result of your calculation is the actual piston clearance Finally compare your calculated clearance to the manufac turer s specification If the clearance is outside specifications the pis ton and cylinder must be resized to make the clearance conform to specification This method is the most accurate way to measure the piston to cylinder clearance Some technicians prefer to measure the clearance directly using feeler gauges Although this method isn t normally recommended it s men tioned here so you ll be aware of it Using this method of measuring the piston clearance the piston without the rings installed is placed into the cylinder bore A blade from the feeler
36. d the piston rings in place The ring lands are the uncut areas between the ring grooves The ring grooves are actually slightly wider than the piston rings As a result the rings can move slightly or float within their grooves The rings are able to actively conform to the cylinder walls while the engine is operating The small amount of space between each piston ring and the bottom side of its groove is called the piston ring side clearance As you can imagine the combustion gases forcing themselves onto the piston get down into the ring grooves and leave behind a residue Therefore to inspect the ring grooves for excessive wear you must first clean the grooves thoroughly When cleaning the grooves re member that the piston is made of aluminum a soft metal Be careful not to dig into the piston and remove any metal especially along the inner sides of the ring grooves The most common tool used to clean the piston ring grooves is an old piston ring Made of a very tough material old piston rings work well because they fit the ring grooves perfectly and therefore won t damage the sides of the grooves If you use an old ring for this purpose break it in half to produce a scraper like edge Then insert the edge into the groove and scrape the residue out After the ring grooves are cleaned the piston can be wiped off and the side clearance for the piston rings can be checked As mentioned earlier this dimension is the clearance betw
37. dition since a gasket is used to help form the seal between the head and the block there may be some gasket material left behind when the cylinder head is removed from the cylinder This leftover material can be removed in the same way you would remove carbon buildup Remember that most cylinder heads are made of aluminum which is a relatively soft metal Be careful that you don t dig into the cylinder head with the scraper or brush when cleaning it After the carbon residue and gasket surfaces are scraped off the head can be cleaned with cleaning solvent Checking for Damage After the cylinder head is cleaned it can be thoroughly checked for any visible signs of damage Check for small cracks or other damage in the area of the combustion chamber Also if cooling fins are broken on air cooled engines the head may need to be replaced When work ing on liquid cooled engines check all the water jackets to ensure there are no obstructions In most cases cylinder heads are very reliable The most common cylinder head problem you ll see is damage to the threads in the spark plug s hole On all four stroke motorcycle and ATV engines the spark plug is threaded through a hole in the cylin der head After the head is removed from the engine you can easily clean and check the condition of the threads in the spark plug s hole Because most cylinder heads are made of aluminum the threads in the head can be easily damaged If the threads appe
38. e A few small blocks of wood under the cylinder head provide the proper support After you ve obtained the proper size driver place it on top of the valve guide Then use a ball peen hammer or press to knock or push out the guide Even though either method can be used on most engines always refer to the service manual to determine the exact procedure for the particular engine on which you re working Inspecting the bore After you ve removed the old valve guide use solvent to clean the bore in the cylinder head where the guide was located Visually inspect the bore for deep scratches or other signs of physical damage If any damage is present the new replacement guide may not fit tightly enough If the bore appears to be damaged the replacement valve guide needs to be slightly oversized that is its outside diameter needs to be larger than the outside diameter of the old guide If you have a replacement guide that s large enough you ll be able to correct the damaged bore by reaming it out to fit the oversized guide Keep in mind that oversized guides aren t available Four Stroke Engine Top End Inspection 39 for all engines and some engines won t have enough metal around the guide bore to allow for enlargement In this case your only choice is to replace the cylinder head FIGURE 20 Removal of the valve guide is aided with a ball peen ham mer and the proper valve guide remover VALVE GUIDE Copyright by American Honda
39. e and ATV industry the single overhead cam SOHC and double overhead cam DOHC designs Camshaft installation is slightly different between the SOHC and the DOHC engine There fore we ll explain each one separately Single Overhead Cam Design 1 Align the T mark on the engine flywheel with the index mark on INDEX MARK the crankcase to verify that the position is at TDC Keep tension on the cam chain while doing this If tension isn t placed on the cam chain the chain may get caught between the crankcase and the cam chain drive sprocket 2 Install the camshaft so the cam lobes are pointing downward Engines with rocker arms above the camshaft must have their cams installed lobes down This ensures that the engine is at TDC compression as the intake and exhaust valves are both closed in this position ENGINE FLYWHEEL Courtesy of American Suzuki Motor Corporation 56 3 Engage the chain onto the camshaft sprocket In our example there s also a locating pin that lines up when installed correctly Make sure the marks on the camshaft align in parallel with the surface of the cylinder head Double Overhead Cam Design 1 Install the camshafts into the cylinder head and install the camshaft caps and bolts For our example first align the T mark with the index mark Keep tension on the cam chain while doing this On engines with bucket style valve depressors the cam lobes should point upward
40. e piston s skirt is larger than the diameter at the piston s crown The service guide shows where to measure the diameter of the piston Figure 8 provides an example The piston must be measured with a micrometer It s a good idea to keep track of the piston s actual di ameter because you can use that measurement when calculating the clearance between the piston and the cylinder walls Once you ve measured the diameter of the piston compare your measurement to the appropriate specification or specification range If the diameter of the piston is outside the specification the piston should be replaced If the piston is within specifications and shows no visual signs of damage you can reinstall it in the engine Four Stroke Engine Top End Inspection 2h FIGURE 8 Measuring a Piston Copyright by American Honda Motor Co Inc and reprinted with permission SPECIFIED DISTANCE Measuring the Cylinder Now we ll measure the cylinder to determine the amount of wear that has occurred on the cylinder walls Movement of the piston and rings within the cylinder contributes to cylinder wear The areas of wear are the locations in which the rings travel as well as the areas in which the piston skirt contacts the cylinder walls Cylinder wear is also caused by the piston rocking on the wrist pin due to the piston tipping slightly during its travel Piston rocking can create a noise known as piston slap Under these conditions the
41. eamer A reamer is a long round cutting tool with cutting edges along its length The tool operates much like a drill bit Unlike a drill bit how ever a reamer doesn t cut on its end it can t be used to actually drill a 40 Four Stroke Engine Top End Inspection hole in a piece of metal The cutting surfaces of a reamer are along its sides The tool is used to remove material only along the inside surface of an already existing hole To ream out a valve guide the reamer is inserted into the hole and turned clockwise until it penetrates the entire length of the guide Figure 21 Because of the typical design of the reamer s cutting edges the tool should always be turned in the clockwise direction turning it in the opposite direction will dull the edges Even when you re backing the reamer out of the valve guide you should con tinue turning it in the clockwise direction FIGURE 21 A Typical Valve Guide Reamer Copyright by American Honda Motor Co Inc and reprinted with permission VALVE GUIDE REAMER The appropriate inside diameter of a valve guide depends on the size of the valve stem The service manual for the engine specifies the proper diameter This allows sufficient clearance for the stem to move through the guide as the valve opens and closes After a valve guide has been reamed any metal particles should be removed with com pressed air Then the area should be washed with solvent Reconditi
42. easure the flatness in several directions across the cylinder head In most cases the best indication of the head s flatness is found when the straightedge is placed diagonally across the head s surface Generally the measurement you should use to compare against the manufacturer s specification is the maximum amount of warp measured at any point on the surface FIGURE 12 Use a straightedge and a STRAIGHTEDGE feeler gauge to check for cylinder head warpage Courtesy of American Suzuki Motor Corporation You can also use a surface plate to check the flatness of a cylinder head A surface plate is a flat piece of material with a perfectly smooth sur face Special surface plates made of very thick metal or granite are used for high precision machine work however for checking a cylinder head a special surface plate provides a greater degree of precision than is actually needed A surface plate good enough for checking a cylinder head can be a simple piece of plate glass such as a window pane Plate glass has a very flat surface and is readily available from most hardware stores After you ve obtained a surface plate lay the cylinder head on it Place the head with the surface that will hold the gasket facing down Then use a feeler gauge to measure any clearance between the surface plate and the head Feel for gaps all the way around the head Finally com pare the largest clearance that you measured against the manufacturer
43. een the piston ring and the inner side of the ring groove Figure 6 This small amount of clearance performs an important function During the power stroke the pressure produced by combustion pushes the piston down the Four Stroke Engine Top End Inspection 25 FIGURE 6 Measuring the Piston Ring Side Clearance Copyright by American Honda Motor Co Inc and reprinted with permission bore Some of the expanding gases are also forced down the side of the piston and behind the floating piston ring The resulting pressure behind the piston ring forces the ring outward hard against the cylin der wall thus helping to better seal the combustion chamber By al lowing the ring to seal better the proper ring side clearance helps the engine produce more power PISTON RING j PUSH IN H A PISTON FEELER GAUGE Note that because a small amount of clearance should always be pres ent a ring tips slightly under normal operating conditions as shown in Figure 7 As the piston goes down the cylinder during the intake stroke the ring tips and scrapes excess oil off the cylinder wall Dur ing the compression and exhaust strokes the piston rises and the tipped ring glides over the oil film remaining on the cylinder wall During the power stroke forces pushing down on the ring cause it to sit squarely providing a better seal and therefore better power The proper clearance between the piston ring groove and the piston ring ca
44. er as well as the cylinder head alignment pins 7 To complete the installation procedure for the cylinder install the cylinder head gasket onto the top of the cylinder Installing the Valves Four Stroke Engine Top End Inspection CYLINDER HEAD ALIGNMENT PIN CYLINDER HEAD ALIGNMENT PIN CAM CHAIN TENSIONER GUIDE Valve springs must be installed so that the narrow pitch of the spring sits on the surface of the head These parts are installed in reverse or der from the disassembly procedures described earlier Always re place the valve stem seals when the valves are removed to prevent any chance of oil seal related problems Note The illustrations used in the following example procedure are courtesy of the American Honda Motor Corporation and printed with permission unless otherwise noted 1 Notice the proper order of installation of the valve assembly The narrow pitch of the valve spring must sit on the surface of the head RETAINER OUTER SPRING S INNER SPRING Four Stroke Engine Top End Inspection 2 Reassemble the valves using a valve spring compressor to hold the springs while installing valve keepers 3 Install the valve keepers or cotters with a pair of needle nose pliers 4 After you ve installed each valve it s a good habit to lightly tap on the valve stem to firmly seat the valve keepers in their grooves 53 VALVE SPRING COMPRESSOR COTTERS
45. er on the valve seat This hammering action dis torts the seat eventually allowing combustion gases to leak past the valve head even when the valve is closed For these reasons all com ponents of the valve assembly must be thoroughly inspected and reconditioned as part of any top end engine inspection or rebuild Inspecting the Valves Before a thorough visual inspection can be performed the valves must be cleaned Anything that comes in contact with a valve such as oil gas and carbon tends to get baked onto the valve surface Ordinary cleaning solvents may not clean the valve completely In most cases the best way to clean a valve is to use a wire brush This ensures the removal of any buildup An ordinary handheld wire brush can be Four Stroke Engine Top End Inspection 35 FIGURE 15 Typical Problems Associated with Valves used but a wire brush in the form of a wheel mounted on a bench grinder can make the job much easier After you ve cleaned the valves with the wire brush wash them in cleaning solvent to remove any leftover dirt particles Now you can begin the actual visual inspection Visually inspect both the intake valve and the exhaust valve This includes inspecting each valve for signs of physical damage and determining if each valve is sized within specifications Valves can be damaged as the result of several conditions Most often though valves are damaged by heat If a valve becomes overheated its edges can me
46. gauge is also inserted into the bore along either side of the piston skirt Two blades are sometimes used one on each side of the piston so the piston remains centered in the bore The various sizes of the gauge blades are in serted until the correct measurement is found The blades that give the correct measurement produce a slight drag when pulled The measurement read off the blades is the actual piston to bore clear ance To make this procedure a little easier many technicians use longer than normal feeler gauges These are available from most tool suppliers The increased length of the blades makes it easier to meas ure the clearance inside a cylinder bore It s usually more difficult to get accurate results using the feeler gauge method than by taking the actual physical measurements of the piston and cylinder 30 Four Stroke Engine Top End Inspection Inspecting the Wrist Pin The wrist pin is a cylinder shaped component of the piston assembly It s used to link the connecting rod to the piston The connecting rod s bearing surface for the wrist pin allows the end of the rod to rotate freely around the pin as the piston travels up and down The wrist pin must transfer each power stroke s downward physical force from the piston to the connecting rod To ensure the wrist pin is strong enough to handle the task the engine manufacturer usually makes the pin of high quality steel a very hard metal For this reason you won t usuall
47. gine sprocket bolt or nut To ease removal put the engine in gear and apply the rear brake firmly before removing the drive chain or drive shaft engine sprocket 9 Remove the drive chain 10 Remove all electrical wiring connected to the engine MAGNETO BFAD WIRES Courtesy of American Suzuki Motor Corporation Four Stroke Engine Top End Inspection 9 11 Remove all necessary control cables such as the clutch cable 12 Remove brake pedal bracket 13 Remove breather hoses BRAKE PEDAL BRACKET MOUNTING BOLTS Courtesy Kawasaki Motor Corp U S A Removing the Engine Depending on the machine an engine can weigh over 300 pounds Therefore it s advisable to use an assistant when taking the engine out of the chassis Note that depending on the motorcycle or ATV the engine is removed from the left or from the right side of the chas sis Check your service manual this information makes engine re moval easier 1 Ifnecessary remove the four bolts securing the removable frame REMOVABLE section 2 Remove the engine mounting bolts The position of mounting bolts varies with different machines Note that some motorcycle and ATV engines have more mounting bolts than others Also some motorcycles have the ability to remove a section of the frame to ease the removal of the engine The mounting bolts of the Kawasaki are ae eee MOUNTING shown in this figure BOLT Courtesy Kawasaki Motor Corp
48. gure 3 Check the piston for cracks or any other signs of surface damage Pay particular attention to the sides of the piston Examine the piston s sides in the areas of both the skirt and the rings these areas are the most common sites of damage One of the most common types of damage to the piston skirt is scoring Score marks are deep vertical scratches A similar type of damage is scuffing Scuff marks are wide areas of wear on the piston that usually appear as shiny patches Scuffing may or may not be accompanied by score marks SCORES DUE TO HIGH HEAT COMBUSTION SCRATCHES CRACKS Scoring and scuffing can be the result of a variety of conditions In most cases the marks are created by excessive friction and heat Un der certain extreme conditions the temperature in a cylinder can ap proach the melting point or weld point of aluminum These very high temperatures can be caused by a problem in the engine s cooling sys tem or excess friction between the cylinder wall and the piston rings Excessive friction is often due to improper lubrication or to the piston fitting too tightly within the bore of the cylinder If you find score marks or scuff marks on a piston try to determine the cause so you can prevent the damage from reoccurring This is one of the times you can take advantage of the notes and observations you made earlier in the disassembly process During the disassembly you should have checked to determine that the
49. hen taking the four stroke engine apart Removing the Spark Plug Remove the spark plug from the cylinder head This relieves engine compression and allows the engine to rotate easily when necessary Four Stroke Engine Top End Inspection 11 Removing the Cylinder Head Cover Depending on the model you re working on there may be either a simple valve cover or a rocker box mounted on the top of the cylinder head The term rocker box refers to engines that have the entire rocker arm assembly and cover case built together as one assembly Valve covers or rocker boxes are both known as cylinder head covers Our ex ample uses the rocker box style cylinder head cover Note The illustrations used in the following example procedure are courtesy of the American Suzuki Motor Corporation unless other wise noted x A iee ft iry 2 Rotate the engine to top dead center laa An on the compression stroke Do this CAPS by watching the intake valve open m and close As the intake valve closes the piston approaches top dead center compression The valve springs fully extend when the valves are closed There should be minimum tension on the rocker 1 Remove the rocker arm cover caps You ll now be able to see the intake and exhaust valves arms 3 Loosen the rocker arm adjusting nut and screw just enough to ensure that all pressure is relieved on the valve springs This ensures valve to rocker arm clearance and makes cover rem
50. ing and closing of SUPPORT the valves in a AREAS four stroke engine Image courtesy of Yamaha Motor Corporation U S A CAM LOBE CAMSHAFT SPROCKET The camshafts used in motorcycle and ATV engines are quite depend able It s rare to find a motorcycle or ATV engine camshaft that s been excessively worn The camshaft in a small engine is normally well lubricated especially in engines that use a high pressure lubrication system When you re rebuilding a motorcycle or ATV four stroke en gine the camshaft should be visually inspected for any signs of dam age Specifically look for any cam lobes that appear to have surface damage Also check the camshaft s ends that are supported in bearings Look for any signs of scoring or other surface damage Motorcycle and ATV manufacturers provide a specification for the diameters of each part of the camshaft Measure those areas using a micrometer and check your measurements against the specifications 48 10 Four Stroke Engine Top End Inspection Road Test 3 Scoring or scuffing on a piston is most likely caused by and What part of the piston is the most frequently damaged If the cylinder is excessively worn what can frequently be done to recondition it for reuse What tool is used to measure a cylinder s bore The difference between measurements taken on the same axis X or Y of a cylinder is called Name two engine conditions that cause the fuel mixture
51. installation of the cylinder head cover before checking the valve adjustment because the rocker arms are installed in this cover DOHC designed engines have the cylinder head cover installed after the adjustment of the valves Note The illustrations used in the following example procedure are courtesy of the American Suzuki Motor Corporation unless otherwise noted 58 Four Stroke Engine Top End Inspection 1 Most SOHC covers require that you seal the surface using a sealer 2 Once sealed install the cover and tighten the mounting bolts in the sequence shown Four Stroke Engine Top End Inspection 59 3 Note that DOHC covers have a gasket installed between the cover and the cylinder head GASKET Courtesy of American Suzuki Motor Corporation Checking and Adjusting Valve Clearance It s important to check the clearance between the rocker arm and valve stem tip after any work is done on the valves and valve assem bly Because the valve clearances may have been changed by the reconditioning and lapping procedures the clearances should be measured and compared with the manufacturer s specifications If a valve clearance is incorrect it needs to be adjusted The rocker arm has an adjusting screw for the technician to make the correct adjust ment of this clearance There are different adjustment methods used for different valve ar rangements Valve clearance is normally checked when the cylinder is at top dead ce
52. is removes any tiny burrs left from narrowing the seat However should you narrow the seat by grinding away its top or its bottom You can decide based on where the valve face makes contact with its seat It should be 44 Four Stroke Engine Top End Inspection centered on the valve face At the same time you can check on whether the valve face makes contact all the way around its seat It s very important that you finish the job with a proper and complete seal between the face and the seat To decide which end of the seat s width you should grind down and to check the seal you can use a technique that involves the applica tion of a blue dye called Prussian blue This is a special dye that can be purchased from most automotive supply stores First remove the grinding tool and pilot from the valve guide Then place a coating of the blue dye on the valve face Insert the valve into the guide and press it in until it firmly contacts the valve seat When the face is in contact apply a slight amount of pressure and rotate the valve one quarter turn in the seat Then remove the valve and look carefully at the valve face The dye mark left on the valve seat indicates exactly where the valve face contacts the seat Ideally the contact area should be in the middle of the valve width Figure 27 FIGURE 27 The valve seat should contact the valve in the center of the valve face Courtesy of American Suzuki Motor Corporation
53. lt or its head can crack If the damage is severe enough pieces of the valve can actually break off Common types of valve damage are illustrated in Figure 15 If you notice any of these types of damage you should replace the valve YYYY A B C D BURNING PREIGNITION CRACKING NECKING Measure the valve margin to detect any signs of distortion The margin is the area between the valve s head and the line where the valve face begins The valve margin is usually measured with a small ruler or with a Vernier caliper A valve with too small a margin won t be able to withstand the heat in the combustion chamber The valve will usu ally crack or burn through and won t be able to seal properly When you re checking valve margins always remember to read the manu facturer s specifications for the motorcycle or ATV engine on which you re working In addition to measuring the valve margin you should measure the valve stem which is the part extending down from the valve s head A normal outside micrometer can be used to measure the valve stem s diameter It s a good idea to measure the valve stem at the top middle and bottom The diameters at all three of these locations should match the manufacturer s specification for the stem s diameter The valve springs must also be measured for their free length to ensure there s enough seat pressure placed on the valve itself This measure ment is done using a Vernier caliper a
54. n be critical If the clearance is too large the ring tips exces sively as the piston moves up and down reducing its ability to seal The excess movement of the ring on the piston may also cause the ring to break If the clearance is too small the ring binds in its groove when the piston heats up and expands After you ve measured the piston ring side clearance compare your measurement with the manufacturer s specification Also because each ring groove may be worn differently you should check the side clearance in all of the piston s grooves 26 Four Stroke Engine Top End Inspection FIGURE 7 Piston Ring Movement within the Piston COMBUSTION Groove CYLINDER SEER A B C RING DURING RING DURING RING DURING INTAKE STROKE COMPRESSION POWER STROKE AND EXHAUST STROKE Measuring the Piston After the piston and rings have been visually inspected you can pre pare the piston for measurement If the piston is to be measured prop erly its rings must first be removed To remove a ring spread the ring open so you can slide it out of its ring groove and off the piston As mentioned previously using piston ring pliers is very helpful although some technicians spread piston rings open by hand A typical piston appears to have a simple shape like a can However looks can be deceiving As mentioned in a previous study unit a typi cal piston is manufactured with a taper That is the diameter at the very bottom of th
55. nder Head The cylinder head must be removed properly to avoid warping or other damage to the head assembly Be sure to inspect the cylinder head assembly after it has been removed Note The illustrations used in the following example procedure are courtesy of the American Suzuki Motor Corporation 1 Remove the cylinder head holding nuts and bolts Loosen the fasteners in a crisscross pattern in two or more steps to prevent warpage of the cylinder head Four Stroke Engine Top End Inspection 15 2 Some models use additional fasteners located in various places to help secure the cylinder head to the cylinder It s necessary to remove these fasteners to allow the head to be separated from the cylinder by lifting it off the stud bolts CYLINDER CYLINDER HEAD HEAD NUTS 3 Remove the cylinder head from the cylinder If it s difficult to separate the cylinder head from the cylinder gently tap on the side of the head with a soft mallet to break the seal between the gasket and the cylinder head 4 Visually inspect the cylinder head and camshaft The bearing surfaces for the camshaft must be smooth and within the clearance specifications stated in your service manual The surface of the cam lobes where the rocker arm rides should have no indentations or signs of wear A more detailed explanation of inspection and repair of the cylinder head and camshaft is given in the next section of this study unit CYLINDER
56. nter TDC At the TDC position on the compression stroke both the intake and exhaust valves for the cylinder should be completely closed As a general rule the valve clearance is greater on the exhaust valve than on the intake valve because the exhaust valve gets hotter producing greater expansion of metal Motorcycle and ATV valves are adjusted when the engine is at room temperature Specifications for valve clearance vary with each model of motorcycle Valve clearance is designed to allow for the heat ex pansion rate of the parts to create a near zero clearance when the engine is at its normal operating temperature Note The illustrations used in the following example procedure are courtesy of the American Honda Motor Company and reprinted with permission 60 1 When the cylinder is at the TDC position check the clearance using a feeler gauge Insert the feeler gauge between the rocker arm and the valve stem to determine clearance until you find the blade of the proper thickness The proper blade size resists with a slight drag when you attempt to pull it from between the valve stem and the tappet If the blade moves without any drag it s too thin if you can t insert the blade between the components it s too thick 2 Adjust the clearance using the adjusting screw and locknut The actual adjustment procedure for four stroke motorcycle and ATV valves is discussed in detail in a later study unit ADJUSTING SCREW
57. oduced in the combustion chamber on the compression stroke Figure 4 The compression is measured with a special gauge inserted in the spark plug hole If the piston rings are worn the gauge displays a pressure reading that s much lower than the manufacturer s specification The reading is low because instead of being compressed some of the air and fuel mixture is leaking past the worn rings and into the crankcase FIGURE 4 A compres sion test can help to COMPRESSION diagnose a problem GAUGE before disassembling a four stroke motorcycle or ATV engine Copyright by American Honda Motor Co Inc and reprinted with permission ATTACHMENT Measuring the Piston Rings New piston rings are measured by fitting them into the cylinder and checking the end gap with a feeler gauge This is done by inserting a piston ring into the cylinder squarely using the piston as a guide After the piston ring is inserted you can then check for the end gap using a feeler gauge Figure 5 Each piston ring should be measured at the top middle and bottom of the cylinder The specification for the proper ring end gap is given in the appropriate service manual 24 Four Stroke Engine Top End Inspection FIGURE 5 Procedure for Checking Ring End Gap Copyright by American Honda Motor Co Inc and reprinted with permission FEELER GAUGE Inspecting the Piston Ring Grooves The ring grooves cut into the sides of the piston hol
58. oning Valve Seats A valve seat is that part of the valve assembly that mates with the valve face Figure 14 Usually a worn valve seat can be reconditioned to get it back into shape The seal formed by the valve seat s precise fit with the valve face prevents leakage from the cylinder when a valve is closed Due to the seat s location near the combustion chamber a valve seat like the valve itself must be able to withstand high temperatures A valve seat must also be able to conduct the heat from the closed valve and dissipate it to the engine s cooling system If a valve seat didn t help dissipate the valve s heat the valve would get so hot it would simply begin to melt Four Stroke Engine Top End Inspection 41 Melting of a valve is often referred to as burning Most often the ex haust valves are the valves that burn Exhaust valves get very hot from passing exhaust gases An intake valve doesn t get quite as hot because the incoming air and fuel mixture tends to slightly cool it When a valve closes the valve face fits closely into the valve seat As a result the heat from the valve head is passed into the valve seat From there the heat can be dissipated to the engine s cooling system to the air in an air cooled engine or to the coolant in a water cooled engine The valve seat s ability to dissipate heat is just as important as its ability to provide a proper seal As you can imagine valve seats must be made of
59. oolant flows out easier if you first remove the radiator cap RADIATOR CAP nn Four Stroke Engine Top End Inspection 3 Remove the coolant drain plug to drain the engine coolant This plug is usually located on the water pump DRAIN BOLT Removing the Engine Body Components While the fluids are draining in the appropriate containers continue to remove all necessary chassis and body components These compo nents include but aren t limited to e Seat side covers e Battery on applicable models e Fuel lines e Fuel tank e Fuel tank brackets To remove the other components complete the following steps Four Stroke Engine Top End Inspection 1 Remove the front and rear fenders This is necessary for engine removal on some ATVs REAR FENDER 2 Remove the coolant hoses and switch connectors on liquid cooled models f RADIATOR gt HOSE Courtesy Kawasaki Motor Corp U S A Four Stroke Engine Top End Inspection 3 Remove the radiator on liquid cooled models The radiator is secured by four bolts in this figure MOUNTING ZOEK 4 Remove the ignition coils on most models 5 Remove the carburetor s 6 Disconnect the crankcase breather hose CRANKCASE M BREATHER HOSE Courtesy of American Suzuki Motor Corporation 8 Four Stroke Engine Top End Inspection 7 Remove the exhaust system The exhaust system is secured by the exhaust pipe holders and nuts 8 Remove the en
60. oval easier CONICALLY RECESSED TOP BOLTS 4 Remove any overhead valve oil lines from the connection at the front or rear cylinder head cover 5 Remove the fasteners that secure the cylinder head cover to the cylinder head These fasteners are located on the very top of the engine Remove the bolts in the order indicated in the illustration Don t remove the conically recessed top bolts 6 Remove the cylinder head cover A Four Stroke Engine Top End Inspection slight tap with a soft mallet may be CAMSHAFT DRIVE CHAIN required to break the gasket seal between the cylinder head cover and the cylinder head Tap squarely with the mallet Don t pry the cover off with a screwdriver as you may damage the sealing surface of the cylinder head The cylinder head cover can now be lifted free and inspected You ll notice that the engine cam timing marks are visible at this time Remember that the camshaft must be timed precisely with the crankshaft to ensure the valves open and close at the correct time Camshaft timing is discussed in detail later in this study unit VALVE SPRING ENGINE CAM TIMING MARKS While the cover is off check for free smooth movement of the rocker arms on the pivot shafts Also check for oil leaks where the pivot shaft enters the cover There are rubber O rings oil seals or sealing washers used where the shaft goes through the cover they may need to be replaced Finally
61. proper amount of oil was present in the engine Also you should have noted any defects in Four Stroke Engine Top End Inspection Zi the lubrication system If you noted a possible source of damage dur ing disassembly and later found marks on the piston you have im portant clues for use in the troubleshooting process In such a situation you may also want to talk to the customer or machine s op erator to find out whether the engine was overheating during opera tion Looking for Oil Residue Engine overheating in addition to causing scuffing and scoring usu ally produces a buildup of oil residue on the piston and the rings Ex treme heat breaks down the viscosity of oil and reduces its lubricating ability When oil breaks down it starts to bake onto the engine compo nents forming a residue that resembles varnish This residue can coat the piston rings and eventually cause the rings to stick firmly to the piston If this occurs the rings are no longer able to seal the combus tion chamber properly Therefore always check to ensure that the rings are free to move on the piston and that both the piston and rings are free of any buildup Examining the Piston Crown Although the skirt is the most frequently damaged site on a piston you must also carefully examine the piston crown If you find any damage try to determine the exact cause so you can prevent the dam age from happening again Damage to the crown is usually the result
62. r of the valve head e Dimension B is the angle of the valve face This angle matches the angle of the valve seat e Dimension C is the width of the valve margin e Dimension D is the diameter of the valve stem 34 FIGURE 14 The Parts and Dimensions of a Four Stroke Engine Valve Four Stroke Engine Top End Inspection MARGIN LOCK GROOVE pam PARTS OF THE IMPORTANT VALVE VALVE DIMENSIONS Valves must operate under a variety of extreme conditions As a re sult certain areas of the valve assembly often show signs of wear or physical damage Because they re located in the combustion chamber valves can reach temperatures of well over 1 000 degrees Fahrenheit under normal operating conditions It s not surprising that they can actually begin to melt under these high temperatures Heat tends to wear away the exposed surfaces particularly the valve heads Heat however isn t the only problem valves face In addition friction between the valve stems and the valve guides produces wear Keep in mind that each valve must open and close for every power stroke in a four stroke engine Because motorcycle and ATV engines operate at 8 000 rpm or more valves must open and close about 4 000 times per minute When a valve opens and closes this fast friction builds up between the valve stem and the valve guide This eventually leads to wear in the stem the guide or both The rapid movement of the valve also tends to hamm
63. r valve head clearance in the four stroke piston These cutaways or pockets are de signed to allow valve head and piston clearance as they both move This clearance prevents the piston from hitting and bending the valve as it opens and closes 18 1 Remove the piston pin retaining rings and slide the piston pin out of the piston 2 You may need to use a piston pin puller to aid in removing piston pins that are too tight to simply push out 3 After the piston is removed from the connecting rod you can remove the piston rings Use piston pin pliers to remove the rings If this proves difficult try to remove them by hand by spreading the end gap open and sliding the ring upward Be very careful not to break the rings when removing them from the piston Four Stroke Engine Top End Inspection PISTON PISTON PIN PISTON PIN RETAINING RING Four Stroke Engine Top End Inspection ike Road Test 2 1 Pistons used on motorcycle and ATV four stroke engines normally have piston rings 2 Why is a scraper ring used on a four stroke engine 3 You should remove cylinder head fasteners in a pattern to prevent warping the cylinder head 4 What special tool is used to help remove the cylinder head valves 5 Four stroke pistons often have in them to prevent the valves from contacting the piston while the valves open and close Check your answers with those on page 63 FOUR STROKE ENGINE TOP END IN
64. rences between removing the engines from these machines but as you ll see there are many similarities as we remove the engine from the chassis We follow the procedures in steps to help you better understand the correct way to remove an en gine from a four stroke motorcycle and ATV FIGURE 1 The Kawasaki GPZ 500S Motorcycle Courtesy Kawasaki Motor Corp U S A FIGURE 2 The Suzuki LT 250S ATV Courtesy of American Suzuki Motor Corporation 4 Four Stroke Engine Top End Inspection Cleaning the Engine and Draining the Fluids Before attempting to do any work on a motorcycle or ATV it s ex tremely important that you clean the engine and the surrounding components thoroughly with a suitable cleaner There are many types of cleaning agents available at automotive parts stores as well as at your local motorcycle or ATV dealership You ll find that disassembly is much easier when you re working with a clean piece of equipment Note The illustrations used in the following example procedure are courtesy of the American Honda Motor Co Inc and reprinted with permission 1 Remove the engine oil drain bolt This prevents oil spillage while you re removing the engine from the chassis OIL LEVEL GAUGE SEALING WASHER DRAIN BOLT 2 If you re working on a liquid cooled motorcycle or ATV remove the radiator cap This is a preliminary step to draining the engine coolant You ll find that the c
65. s exceed allowable lim its in many cases the cylinder may be bored or recut to a new size and fitted with a new and larger piston and ring set Not all cylinders can be bored check the appropriate service manual to determine if the cylinder is capable of being bored Cylinder Boring Boring a cylinder is a job that requires the use of special machine tools and is normally done by a specialist Boring can be done at most automobile machine shops as well as at many dealerships Measuring the Piston to Cylinder Clearance A piston expands as its temperature rises Because the metal of the piston typically expands more than the metal of the cylinder wall some clearance must be allowed between these components when both are cold This clearance is called the piston to bore clearance or piston clearance for short The proper piston clearance for an engine is given in the appropriate service manual Four Stroke Engine Top End Inspection 29 Piston Clearance A Critical Dimension If the clearance is too small the piston fits too tightly in the cylinder when the engine heats up resulting in excessive friction Friction be tween the piston and the cylinder can be so great that the piston seizes in the bore That is the piston may wedge itself so tightly into the cyl inder that it can t move up or down If this occurs the engine stops running and the starting mechanism won t be able to rotate the en gine You may be able to free a seize
66. s illustrated in Figure 16 36 FIGURE 17 A Typical Valve Grinding Machine Courtesy of Sioux Tools Inc Four Stroke Engine Top End Inspection FIGURE 16 Using a Vernier Caliper to Measure a Valve Spring Courtesy of American Suzuki Motor Corporation Refacing Valves If you inspect the valves and find that they re in good condition they can be reused in the engine However all valves will experience some wear and distortion from use Therefore before you reuse a valve the valve face should be reconditioned Be sure to verify that the valve you re refacing does not contain stellite If it does and it s recondi tioned the valve will wear out very rapidly as the stellite coating is very thin and is used to harden the valve surface The valve face of a non stellite coated valve can be reconditioned to make it smooth and give it a uniform shape The process of recondi tioning the face of the valve is commonly called valve grinding or refacing A machine like the one in Figure 17 is used for this process A refaced valve seals property when reinstalled in the engine Keep in mind that most modern engines come with stellite coated valves and therefore shouldn t be resurfaced If these valves show excess wear they need to be replaced HOSE FOR GRINDING OIL Four Stroke Engine Top End Inspection 37 Inspecting Valve Guides Now that the valves have been inspected and deemed acceptable we can move
67. the edge of the rings usually mean that dirt or other debris has been getting into the en gine This may indicate a faulty air filtering system Piston rings should be replaced when an engine is taken apart Nor mally rings that are reused won t seat in properly resulting in poor engine performance You may recall that when new piston rings are installed in an engine they must wear themselves into position against the cylinder walls to form a tight seal Once this process of seating in has occurred the rings lose the ability to do so again That is if old rings are reinstalled in an engine they won t be able to con form once again to the cylinder walls and make a tight seal Without a tight seal the combustion gases leak past the rings This reduces the amount of horsepower the engine can produce In addition oil from the crankcase seeps past the rings and into the combustion chamber The engine thus consumes larger amounts of oil Oil that enters the combustion chamber burns along with the air and fuel mixture Any oil burning in the combustion chamber is revealed by excessive exhaust smoke as the engine runs Worn piston rings are usually bright and shiny at the point where the edge contacts the cylinder wall Worn rings can also be detected by performing a compression check on the engine before it s disassembled A compression check is a simple test that measures the amount of Four Stroke Engine Top End Inspection 23 pressure pr
68. the marks appear on the alternator rotor and the cam chain sprocket Four Stroke Engine Top End Inspection 55 Aligning Timing Marks The alternator rotor is used to generate electricity It s keyed to the end of the crankshaft As a result of its positioning the rotor can be used to indicate the position of the piston It s also used in connection with timing the ignition system There s a timing mark on the stator or engine case Turn the rotor until its timing mark meets the stator When you align the rotor with the stator using these marks the crankshaft is usually in one of three specified positions T which re fers to Top Dead Center TDC F which refers to the ignition firing point at engine idle or II which indicates full ignition advance When you know the degree position of the crankshaft you must get the correct corresponding position for the camshaft Find a timing mark on the camshaft sprocket The mark on the camshaft sprocket is usually aligned with a timing mark on the cylinder head Then insert the camshaft into the sprocket and rotate the camshaft until it fits into the sprocket properly As indicated earlier there are other methods of timing In a gear driven overhead cam where you don t have a sprocket and chain one tooth on each gear is marked for timing align ment We ll now explain the procedures for timing the camshaft to the crankshaft on the two most popular engine designs used in the mo torcycl
69. y see much wear on the pin itself However to guarantee that a wrist pin isn t worn measure the pin s diameter with an outside micrometer and compare your measurement with the specification given in the service manual Figure 11 FIGURE 11 Measure the wrist pin in three locations using an outside micrometer Courtesy of American Suzuki Motor Corporation Inspecting the Cylinder Head In addition to containing the valves the cylinder head is the component that seals the top end of the cylinder The cylinder head is usually attached to the top of the engine cylinder by several bolts A gasket between the head and the block helps create an airtight seal Because the cylinder head must seal off each cylinder the head must be in good condition and free of cracks and warps Before you can accurately determine the condition of a cylinder head you should thoroughly clean it with a cleaning solvent Removing Carbon and Gasket Material Buildup The combustion area of the head may require some special attention when you re cleaning the head Carbon buildup from the combustion process may be present on the surface Carbon buildup is a hard resi due that s often found on surfaces exposed to the burned air and fuel mixture Because the residue tends to be quite stubborn you ll probably need a wooden scraper a putty knife or a wire brush to remove the buildup from the cylinder head Four Stroke Engine Top End Inspection 31 In ad

14 4 Stroke Top End Inspection

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