The Service Dept
THE SERVICE DEPT
SHIMS: A SILLY MILLIMETER OR TWO CAN MAKE A DIFFERENCE IN PERFORMANCE AND RELIABILITY.
Art Jensen
There is a way that you can make your motorcycle run smoother, quieter, faster and longer. There are only two catches. The first is that there is more involved than pouring some magic compound from a can. The second catch is that everything on your bike may already be properly shimmed. But because that is probably not the case, a lesson in proper shimming techniques is in order.
One word of caution: If you have never successfully taken apart and put back together the parts being discussed, don’t try to do it now. If you forget where a part went, it won’t matter much that it’s properly shimmed.
DEFINITION
What is shimming, anyway? It’s making parts fit together properly by adjusting their positioning with spacers usually called shims. A table with legs of unequal length is a good example of something that needs shimming. In order to make it level and keep it from rocking, you may fold a piece of paper to insert under a shorter leg. The paper would be a type of shim.
How can you tell if something is improperly shimmed? If the parts appear to rattle around from too much clearance, or bind up because of too little clearance, or fail to mesh properly because of improper positioning (too far to one side or the other), then they may need to be adjusted or repositioned by putting in, taking out or moving around shims.
FACTORY SPECIFICATIONS
Many service manuals give clearances for various parts of the motorcycle. Examples of this are: “Crankshaft end play: .003-.005;” or “Camshaft end float: .01mm + .OT.OOömm.” End play and end float are the same. They are terms used to indicate the amount of in and out movement a shaft has. If you grab almost any transmission countershaft on a Japanese motorcycle, push and pull on it, you will probably feel and perhaps hear a definite clunk as it moves as far as it can go in and as far as it can go out. That movement is end play. When a specification is given for the amount of end play, it will be in decimal fractions of an inch or millimeter.
TOOLS
In addition to the usual tools needed to work on the motorcycle, special measuring devices are needed. I find it helpful to have a dial indicator, a vernier caliper, a micrometer and a set of feeler gauges. The easiest way to measure end play accurately is with a dial indicator. The one used to check ignition timing will work fine. The only hard part is figuring out how to rig it so that it will be in a position to bear on the end of the shaft. If you don’t have a set of clamps and holders, don’t worry; a couple of pairs of vice grip pliers and a bit drill rod will do the job.
HOW TO MEASURE END PLAY
When the dial indicator is in place, push the shaft in until it stops moving. Adjust the dial to zero or remember what the dial is reading. Pull on the shaft until it stops moving. If the shaft fits tightly in its bearings, you may have to apply leverage. Be careful not to disturb the dial indicator and always take several readings.
Another way to measure end play is with vernier calipers. Simply put the base of the vernier caliper, or the shaft and the depth indicating rod, against the case, inner bearing race or seal (being careful not to measure seal flex). By measuring the difference between “shaft in” and “shaft out,” you’ll have the amount of end play.
Another method of measuring end play is to rig a very rigid wire from some part of the case (like a case screw) to the end of the shaft. When the shaft is pulled out, the wire should barely touch it. When the shaft is pushed in, you can measure the distance it moved with a feeler gauge.
A final technique that can be used if you have nothing else available is the “eyeball method.” If you jerk the shaft back and forth, you’ll probably notice a line where the seal or inner bearing race has wiped the shaft clean or polished it. By placing feeler gauges on the shaft to measure this space, you can make an intelligent guess as to the amount of end play.
SHIMMING WHEELS
When it comes to shimming, the wheels are probably the easiest to work with, yet the most overlooked parts of the motorcycle. Do the bearings bind up when the axle is tightened? When everything is tight, do the hubs have end play on the axles? Are the wheels centered in the forks or swinging arm? In other words, are they too tight, too loose, or in the wrong place? Improper spacing of wheels and wheel bearings can cause parts to wear out rapidly and make for poor handling.
If the wheel binds up when the axle is tightened, it usually means that the pipe-like spacer that fits between the wheel bearings is too short. It may have been crushed or worn. All you have to do to cure this is measure the distance between the two bearing inner races, then make the spacer that length either by adding to it (inserting shims) or getting one the proper length.
If a wheel has end play when the axle is tight, and the wheel bearings are not worn out, the center spacer may be too long. In that case, you can carefully cut it to the proper size.
If a wheel that is laced correctly is not centered in the frame or forks, you can move it to its proper position by use of shimming technique. Simply add to the side that’s too close and subtract from the side that’s too far away. On most slip-through axles, like those usually found on front wheels, you can just add to or subtract from the side opposite the axle shoulder. This is because the shoulder can move closer to or farther from the center of the forks as it slides freely through the leg.
The biggest problem usually encountered in wheel spacing is the fact that when you change one thing, you change others whether you wanted to or not. In other words, if you reposition your rear wheel, you may cause your sprockets to be misaligned. In that case you would have to increase or cut down the distance between the hub and sprocket. This applies to every part of the wheel. Always check the brakes, chain alignment, speedometer drives, sprocket bolts, etc. after any shimming operation.
SHIMMING CRANKSHAFTS
The theory of crankshaft shimming is quite simple. The crankshaft should be spaced so that the top of the rod is centered in the bore. The crankshaft should have enough end play to run freely but not so much as to allow the crank to drag against the cases or misalign parts that run off of it. However, it’s not usually that simple. One of the biggest problems is getting the shaft to move freely enough in the bearings to measure end play. Sometimes a little heat can be applied to the inner races to make them expand. Once this is done, the crank can be moved by careful use of a lever (like a screwdriver) between the case and crankshaft. Some motorcycles require special tools to extract the crank from the cases. On this type, you can sometimes use the tool to carefully push the crank first to one side, then to the other. Once you are able to move the crankshaft, measurement is made by using any of the methods already described.
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As with wheels or anything else that can be shimmed, there are only three ways that crankshaft spacing can be incorrect. The first way is too little clearance. This causes the same problems that occur any time bearings run in a bind: friction losses and premature bearing failure. Too much end play is the second problem, and it can cause the crankshaft to float in and out as the engine runs. As the rod goes in and out of center in the bore, the result may be unnecessary wear and strain on the rod, rod bearings, piston assembly and cylinder bore. Too much end play can also cause improper alignment of primary gears on sprockets, alternator flywheels and camshaft drives on fourstroke engines. The third spacing problem is improper positioning of the crankshaft, which can be responsible for many of the symptoms associated with too much end play.
Special care must be taken whenever the crankshaft has been replaced or rebuilt. These cranks are more than likely a few thousandths of an inch wider or narrower than they were before. It is often wise to assemble the cases with only the crankshaft, using a few screws to pull the cases together. The position of the crankshaft and amount of end play càn then be meásured. Remember to use a gasket when measuring if one is to be used in final assembly.
There are two basic kinds of crankshaft shims. One type fits on the crankshaft itself, between the inner race of the bearing and the flywheel. The other, a much less commonly used type, fits between the outer race of the bearing and the main case or a seal holder that bolts to the main case.
SHIMMING TRANSMISSIONS
The most tampered with, as well as the most difficult, area of shimming is the transmission. The basic rules of not too much or too little end play, and the proper positioning of parts, are the same. The only real complication is the number of parts that have to work together. This, plus the fact that in most motorcycle gearboxes many of the parts change position as the gears are selected, makes for many more variables than are encountered when shimming a wheel or crankshaft. Each time a part changes position, it affects the relative position of the parts around it. The actual shimming process takes much thought and a little practice. However, a properly shimmed transmission shifts better, is stronger, and lasts longer than most transmissions as they leave the factory. The reason most transmissions aren’t shimmed perfectly at the factory is, simply, that the process is so time-consuming; and production time adds to production cost.
The three kinds of gearboxes normally found on motorcycles are the dog-, balland key-engagement types. The one most commonly used, the dog-engagement type, is unfortunately the most difficult to explain, so on to the other types first.
Ball-engagement and key-engagement transmissions work on exactly the same principle. One of the shafts in this type of gearbox has all of the gears rigidly mounted on it. The other shaft has all of the gears mounted on it so that they can spin freely. When the transmission is in neutral, all the rigid gears spin all the non-rigid gears, while the shaft that runs through the nonrigid gears stay still. To engage the shaft, something has to lock one of the non-rigid gears to it. In the key-shifter type, a T-shaped key rides inside the shaft, with its ends sticking out of tracks that run the length of the shaft. The base of the T is a shaft that, when pushed or pulled, moves along the tracks and engages in slots that the gears have in their inner surfaces. When the key is in the slot, the gear is locked to the shaft. When the key is between two gears, all of the gears spin freely. The transmission is then once again in neutral. The space between the gears has to be at least as wide as the width of the shift key, or the key will be able to engage two gears at once, which is the one thing a transmission can never do.
The ball-type engagement method is the samé as the key-type, except that a spool instead of a key is attached to a rod that slides it back and forth inside the shaft. There are balls in holes in the shaft that are forced out into grooves in the inside of the gears. As the spool passes through the shaft, each time it reaches a circle of these balls, the diameter of the spool forces them into the grooves in the transmission gear. When the spool isn’t pushing them out, they return toward the inside of the shaft. Again, the shaft between the rows of balls has to be wider than the part of the spool that forces them out.
Proper shimming of this type of gearbox is really rather simple. On the shaft that has all the rigid gears, one must check to make sure that the gears mesh with the gears on the other shaft. The only other thing necessary is that the shaft have a proper amount of end play. The shaft with the non-rigid gears is a bit more difficult. The shaft itself is usually shimmed independently of the gears. In other words, the shaft without any gears on it can be checked for end play if the cases are bolted together. The gears themselves must mesh with their mates on the rigid shaft. In addition, the center of the T bar or shifting spool must be in the center of the gear when that gear is selected. Also, make sure that there is enough room between the gears so that the T bar or shifting spool cannot engage two of them at the same time. There has to be a neutral between each gear.
If the transmission in this design is easy to shim, then the shifting mechanism oftentimes more than makes up for it. The only way to properly approach it is with the factory service manual for all the specifications and a good deal of patience and knowledge. However, remember that there are only three ways each part of the mechanism can be improperly set. It can be wrong only if it is too tight, too loose, or in the wrong place.
The dog principle of gear engagement differs from the balland keytypes in that the gears are engaged by dogs (protruding metal pieces) that stick out of the side of a gear. The dogs slide into holes or other dogs on a neighboring gear on the same shaft in order to couple the gears together. The pairs of gears opposite each other on the two shafts are always meshed. One gear of the pair spins with the shaft. It is either splined or keyed to one shaft. The other gear spins freely on its shaft. To engage that gear, another gear on the same shaft must move over or be moved into by the freely spinning gear. That gear must spin with the shaft. The dogs on the gears then engage with each other, locking both gears to the shaft. This enables power to be transmitted from one shaft to another. A key to understanding how this type of gearbox works is to remember that when the gearbox is in neutral, of each pair of gears, one gear will be locked on its shaft and the other will spin freely on its shaft. In other words, each turning gear will have as its mate a freely spinning gear.
The shift forks that move these gears back and forth so that they engage or disengage are moved by ramps on a shift drum. The farthest a fork can move is the distance that the ramp on the shift drum can move it. It makes sense that this limited amount of movement be used to best advantage. That is really the purpose of shimming the gearbox.
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The best way to start is to set the amount of end play of all shafts, and the shift drum, if they are not held in place. Quite often the clutch linkage and/or clutch pulls the shaft to one side of the case. Also, the shift drum is often held in place by a plate or keepers. Once the end play is adjusted, the transmission is assembled in one of the cases. Usually, on vertically split cases, it is assembled on the shift-drum side. Once this is done and all the shafts are in their proper positions, shift the gearbox to neutral. The gears should all be equally disengaged. In other words, all the gears that are moved by shift forks to the left and right should be in the center of that amount of space. This is because the shift ramp should move the shift fork just as far to the right of center as it does to the left.
When this has been adjusted, the gearbox should be slowly shifted through its entire shifting sequence. Note how far each set of dogs engages. The object is to make the dogs engage as much as possible without “bottoming out.” If the dogs bottom out, the shift fork will wear prematurely. By moving shims around, you can often move the gear that is to be engaged closer to the engaging gear (the one held in the shift fork). Be careful not to remove any thrust washers that may protect clips or bearings. Also, it is often possible to cut down some of the gear so that it can be shimmed closer. In addition to the possibility of bottoming out the dogs, there is another limiting factor determining how close together the gears can be. Each time a gear is engaged, the previous gear must be fully disengaged. Two gears must never be engaged at the same time. Ideally, when the transmission is in neutral, each gear should be equally disengaged. As the transmission is shifted through the range, each gear should come within a few thousandths of an inch from bottoming out. Finally, each gear should begin engaging the instant the previous gear fully disengages.
The foregoing description applies to a typical Japanese transmission. Most other dog-type transmissions are variations that really use the same principles. This includes the dog-wheel type found in Spanish motorcycles. In any dog-type transmission, care must be taken to ensure that each gear in the shift sequence engages and disengages fully.
MISCELLANEOUS SHIMMING
Clutches, kickstarter mechanisms and primary drive assemblies are other parts that can be made more efficient by proper shimming. In a chain-driven primary drive, care must be taken to ensure that the sprockets are aligned. The teeth of gear-driven primaries should engage fully.
There are too many types of kickstarter designs to discuss them all, but here are a few hints. In pawl-types, make sure that the pawl is shimmed so that it engages as much of the width of the kickstarter gear as possible. On toothed ratchet types, make sure that the engaging teeth engage as soon as possible in the stroke of the lever. However, it is essential that they disengage completely when the lever returns. Kickstarter idle gears will last longer if they are shimmed so that the whole width of the tooth is meshed with its mates.
One part that deserves special attention is the clutch. Quite often the clutch hub and the clutch basket will bind together when the clutch nut is tightened down. This usually happens to engines that have a lot of time on them. Because of wear, the clutch hub seats against the clutch basket/primary gear assembly, causing them to wear against each other. Some manufacturers can supply different thickness shims to go between the clutch primary gear and the inner race of the mainshaft bearing. By fitting a thinner shim, or grinding a bit from the shoulder of the clutch gear where it fits against the bearing, the sleeve or shoulder that holds the clutch hub away from the clutch basket is again long enough to do its job. When the clutch-hub nut is tightened, the hub should spin freely without contacting the clutch basket. The clutch basket/primary gear assembly should have Ht tie or no end play.
CONCLUSION
The purpose of this article was to inform you of the basic theory of shimming. When all is said and done, it once again comes down to those three basic rules: 1. Parts must not be too tight. 2. Parts must not be too loose. 3. Parts must fit in proper relation to one another. These three rules apply to every part on the motorcycle. The only variables are the particulars of whatever assembly needs to be shimmed. As long as you understand exactly how something works, you can figure out if it is working properly. By applying the three rules, the mechanic can bring each part of a machine to its fullest potential. gj]
