Subject: rear end problems & outer bearings
Date: Mon, 15 Nov 1999 09:58:38 -0500
From: Ken Rolt <
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To: 9C1 Mailing List <
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To be brief and for those of you new to the subject, the GM 8.5-inch rear end is lubricated by gear oil (lube). The lube is intended to both lube and cool the differential (ring and pinion gears, sidegears, axle splines, L and R diff bearings) as well as the L and R outer bearings that are next to each rear brake.
How the differential is coated with lube is easy to see: the fluid level in the rear end is such that the ring gear dips into the fluid level and splashes it all over the interior of the rear end.
How the lube gets to the outer bearings is NOT easy to see, but easier to describe. The rear end differential cover has a "shelf" stamped into it, so that lube flung from the ring gear to the inside of the back cover, gets redirected by the "shelf" to both the left and right sides of the diff housing. If you had the rear diff cover removed, you would see two large circular holes in the axle housing. The shelf is designed to direct lube into each of these two holes. The lube then pours onto each axle shaft. Most of the lube then drips off the shaft(s) and onto the floor of the axle tube, but some of it still clings to the axle shafts. The rotation of the axle shafts causes the clinging
lube to creep along the axle shaft inwards (back to the diff) which helps lube the side gears, and the other part creeps outward on the axle shaft toward the outer bearings. That's how it is *supposed* to work.
BUT GM stopped, a handful of years ago, using a diff cover gasket with mating circular lube holes so the lube path to the outer bearings via the axle shaft path was eliminated. Only random splashing within each tube, or frequent vehicle turns, would cause the outer bearings to be lubed often.
What is the result? The outer bearings get too hot and they begin to heat discolor the hardened bearing surface on each (outer) axle, and eventually the axle shafts become pitted. Only then would be BEGIN to hear funny rear end noises, so this is one of the problems that needs to be checked BEFORE you have a problem.... especially on a used police car like the 9C1.
When I bought my '94 9C1, I had the axles pulled and checked. The driver's side axle was ok but the pass side was pitted at the bearing surface. I have heard worse stories where the axle actually becomes grooved. In my case, I had both
bearings and seals replaced, as well as installing a new pass side axle. I made sure that the dealer used a gasket (Felpro RDS 55028-1) with holes to allow the axle shafts to be properly lubed. I have since drained and refilled the diff,
and did my own gasket install. There is also a new GM gasket which supposedly allows the lube holes to be partly or fully opened, and lube correctly, but I have yet to install it myself. Joe Nerz's post on the new gaskets is attached below. I believe that Joe Palmer, among others, put pix up on the web comparing the gaskets (old GM, Felpro, and new GM).
In the crude survey I did over a year ago, among 9C1 owners that did any rear end work, *most* of the time it was the passenger side outer bearing and axle that was shot. I surmised, from working on my own car, that the reason is that
the pass side bearing is much more lube-starved than the driver's side. This is due to the ring gear being located on the driver's side of the car centerline, so it can much more easily splash-lube the driver's side outer axle but it cannot do the same for the pass side. Why? Because of the large obstruction in the way: the differential itself.
---- Tip for New Owners ----
If you have a B or D-platform car (Caprice, Impala, 9C1, Roadmaster, Custom Cruiser, etc) and you've never checked the rear end DO IT NOW. If you are tool handy, study the FSM and do it yourself. I'm not a rear end expert (Roswell, no
funny remarks!!!) so I had someone do the work for me the first time. The passenger side axle is the one that should be checked first, by draining the diff, unbolting the diff cover, removing the c-clip for the pass side, and then removing the pass side axle to inspect the bearing surface. If it looks ok,
then the driver's side is probably ok too. If the axle shows any sign of stress or grooving, it will have to be replaced. In that case, you should have the driver's side checked as well, and both the seals and outer bearings should be replaced. That would also be a good time to have the rest of the rear end
checked for problems.
If you have a G80-equipped car, this is also a good time to have the posi checked out and possibly rebuilt.
Other items relevant:
1. if the diff fluid is low, old, or both, inadequate lube and cooling will result. This would affect the whole rear end, not just the outer bearings.
2. a leaky pinion shaft seal is often the cause of low diff fluid, and hence heat-related failures.
It has never been proven what causes the axle and bearing failures on B and D cars; the two main clulprits are the leaky pinion seal and the lack of lube holes in the rear end diff cover gasket. Both can cause problems.
But I'll put my money on the bad gasket, because I've seen/read failures on plenty of cars with good pinion seals and adequate rear end fluid. Hence outer bearing problems and axle bearing surface failures can only be caused by the
lack of lube, especially on the pass side.
HTH. - Ken mailto:
[email protected]
