Guide to Diff Shimming -
11.01.2010, 11:32 AM
First you will need some shims, depending on what size the pinion shank is will determine what size internal diameter shim you need to correctly shim the pinion. For the diff itself, you can use shims that will fit between the bearings and the cup/crown gear. For diffs with fixed drive cups (most 1/8 scale) you may find it easier to get shims that are the same outside diameter as the bearings (most 1/8 scale stuff uses 8x16 bearings, so 18mm OD shims with 15mm id work ok) and you can shim between the outside of the bearings and the diff case/carrier.
1st step is to assemble the diff and pinion in the case and attempt to see if the pinion is engaging the crown gear (also called ring gear) properly. You can add shims between the pinion and the inner bearing to place the pinion closer to the diff cup. Basically what you want is complete engagement of the ring and pinion teeth. Then I like to install the cup or cvd to the pinion shank and check for play. You may need to add shims between the cvd/cup and the outer bearing. This will eliminate any "walking" of the pinion, and thus keep the ring and pinion teeth in correct alignment. Another plus is that the bearings will last longer. You want the pinion to spin freely, but not be able to push it in or out of the carrier.
2nd step is shimming the diff side to side. This is where most problems occur. If you are starting with new gears you want them reasonably tight, as they will wear in and loosen up over time. If you are starting with used gears you can set them looser, as they are already worn together.
Side note, always change both the ring and pinion out if either breaks, cause using a new gear with an old gear will not last very long, and usually ruins the new gear.
Basically all you do is add shims to one side or the other of the diff to move the crown gear side to side. Best to have clean gears with no grease on them. With new gears you may have a few tight spots as you rotate but the gears will wear in. Also when you shim the diff, make sure you have it shimmed so that it cannot move from side to side in the carrier, this will keep the mesh tight, and not allow any slop, which causes accelerated wear. So you may need to add shims to both sides of the diff, keep this in mind.
Now here is the tricky part. You need to "feel" the gear mesh when you add the shims to the diff. So it is best to get the diff simmed so that it does not move side to side in the carrier, and put all of those shims on the cup side of the diff. This will place the ring gear as far away from the pinion as it can get. You can then rotate the pinion and see if the mesh is smooth. If it is smooth then hold the diff outputs and turn the pinion back and forth to feel for slack. If you do not feel much slack you are good to go. If you do feel slack, move 1 shim from the cup side to the ring gear side and try again. Keep doing this till it feels good, then swap 1 more shim over. That should make it too tight to turn easily. If so, put that shim back on the cup side and you have a correctly shimmed diff. I say to do the last part as a double check on your "feel". If it is too tight you know it was just right, if it still turns smoothly you know you had it too loose.
If these are new gears you will likely have some spots that are a bit tight when you turn the pinion. It is always good to rotate the pinion at least 4 times during each step as you want to get a feel for the entire revolution of the ring gear. Also if you have new gears you may want to run them for a few packs, the pull them back out and see if they still feel smooth and the pinion does not have much play when you hold the diff outputs and try to turn the pinion. The gears will wear in, and you may need to adjust the shims on the diff to tighten them back up. This is where very thin shims come in handy, as you can make small adjustments. Keep the gears well greased, and you will get good life out of your ring and pinion gears.
Shims are available in various thicknesses, OFNA has a great selection, as does HPI. If they offer 2 different thicknesses of shims it may be wise to get both, as that will give you some options.