Many people are very scared of bevelboxes. Because some manuals, most
noticeably the Haynes book, say that anything to do with the overhaul
of bevelboxes should be left up to experts.
While shimming up a bevelbox is not something I'd recommend to your
average owner there is nothing at all to prevent examination of the gears or replacement of the seals by owners, or, the adjustment of the pinion bearing preload.
Once the box is off the swingarm the pinion carrier can be removed from
the main casting. This part is the black flange that is between the
aluminum casting of the box and the swingarm itself. It can sometimes be a bit of a pain to get moving. Probably the best way is to hold the box upright by the pinion shaft a couple of inches off the bench and give the box itself a few vigorous blows with some sort of soft hammer. If this fails to work a bit of judicious prying with a small screwdriver between the box and the carrier will get it moving. It can then be withdrawn from the casting and the pinion teeth examined.
The pinion teeth are almost always the first to show wear simply
because they work several times as hard as the crownwheel teeth. Most common damage is either chipping at the case hardening at the base of the teeth caused by poor mesh or dirty oil or a very high polish on the teeth caused by water in the oil. While highly polished teeth can last for some time chipped teeth will require the replacement of the crownwheel and pinion set.
The commonest cause of bad meshing is that the pinion bearings have
worn slightly so there is end float in the bearings. As the pinion is
rotated there should be no movement other than the rotation. Grasp the carrier in one hand and the pinion shaft in the other if it can be rocked in the bearings or can move backwards and forwards the bearings have worn and there are two choices of action.
1.) The big nut on the pinion can be undone the outer bearing removed
and one of the very thin preload shims between the spacer and the bearing can be removed and the bearing re-installed and the nut re-torqued. When this is done the bearings should be checked by spinning the pinion. Any binding and either too much shim has been removed or the bearings will need replacement.
2.) Replace the bearings, a procedure that is virtually identical to
doing steering head bearings apart from the fact that the bearings have a spacer and shims in between them to pre-load them. When re-assembling after replacement the bearings once again must be shimmed for preload so that they neither bind nor have slop.
Note. All bearings are made to ISO standards and especially if you use
bearings from the same manufacturer it is extremely rare to have to re-shim the pinion depth. This is set with a thicker shim that goes between the pinion head and the bearing it's pressed into. If you do replace the bearings make sure that this shim is not left out, it's vital.
If you have to replace the inner bevelbox seal or need to strip the box
to helicoil damaged drain or filler plugs this is very straightforward. as long as you rebuild the box with fresh gaskets and the same crownwheel shim and torque the bolts to the correct torque setting than the depth of the gears engagement won't change.
It's only if the crownwheel and pinion are swapped that a really careful re-shim is required. Needless to say this should be done in clean conditions and care should be taken not to allow filth into the box or it's bearings but it's not difficult and neither is there some sort of black magic involved. Finding out about problems with your bevelbox early is far better than waiting until it goes *pop* when not only is there danger involved, (If a tooth breaks off it can jam the back wheel or destroy the casting.) but it's also bloody expensive.
Go on! Do it yesterday!!!!!
Regards,
Pete Roper
motomoda@austarmetro.com.au
Moto Moda
23,King St.
Bungendore NSW 2621
Australia
