OK. When I rebuilt the rear end in my '79, I had the differential rebuilt by what I believe to be a reputable shop. The clutch pack they ordered was upgraded to include some ceramic parts. Since the rebuild, I have a banging sound in the diff when I turn from a stop. It becomes more frequent and pronounced as the lubricant heats up. I took the car on a 4 hour road trip and it sounded horrible by the time I arrived.

I took it back to the shop yesterday and they put more additive in the lubricant. I now have about 10 oz of additive in the differential. I did 10 figure eights that afternoon and things seemed OK, but after driving for an hour or so, the banging returned, though intermittenly. I also have a shudder when accelerating straight from a stop that seems to increase as the oil heats up.

So I have a few questions. How long does it take the new additive to affect the performance of the diff? Should I do more figure eights? Second, has anyone had experience with the "upgraded" cluch pack? Perhaps it requires different lubricant ratios. The shop told me there were no special instructions with the pack. Finally, are there adjustments the shop could have gotten wrong that are the source of the noise? Any help would be greatly appreciated.

 

If the shop rebuilt the differential, of course everthing back there was removed and replaced. Maybe go back and look at the basics- namely the pinion snubber and bushing set. Make sure the two snubber bolts are sufficiently torqued and pinned to the diff, and look at both the upper and lower bushings.

 

This may help you down the road but remember when the factory builds a car they build it to run, run, run and run. I always rebuild as close to factory as possible, unless racing which is another issue in itself. I used Bairs to rebuild my diff. They have worked on Vettes for many years and I have had no problems.

I had a friend help me change the rear end gear which was noisey, then called Bairs. no problems.

You may want to call them and see if they can help you. Good people

 

I didn't mention that I took the diff out and brought it to the shop. I replaced the snubber bushing but didn't know there was a torque value for the snubber bushing bolt. I have the torque for the bracket (50 ft lbs), but not the snubber bolt. If you have the torque value for that, I could check it and eliminate that as a cause. Thanks.

 

Is the additive compatible with ceramic clutches ?

 

I don't know. I was hoping someone on the forum would have some experience/information on that. I've looked and can't find anything on that. The shop said there was no special instructions. I've read some posts where people have used other than 80W 90, but I don't know if that's appropriate in this case. Any info would be greatly appreciated.

 

Try sending tracdogg2 a PM ,he'll know the story on the ceramics.

 

I'm not aware of any ceramic parts available. Most likely they are talking about carbon fiber clutches since they the only upgrade (downgrade) for the stock eaton posi. The fact that the shop installed parts other than stock replacement shows that they are inexperienced with them. Carbon fiber clutches must use the 400# preload spring kit to have any chance of working. But the kit won't fit if the spider gear backlash is set properly. You have to open up the backlash excessively to install it. This is why Eaton went to the new design spider gears. Unfortunately they made them out of powdered steel.
Very few shops know how to rebuild a corvette diff. The old rule of re-using the factory shims like the books say does not apply. All C2-C3 diffs were built with NDH bearings which haven't been available for many years. TimkenUSA bearings are the best replacement but do not set up the same. Everything will be a few thousands off. The bearing preloads will be different and the gear pattern will be way off.
New clutches have to be shimmed to set the spider gear backlash. Incorerect lash will create all sorts of problems.
Without seeing pics of the diff it's hard to say what problems it has. If it does have the carbon fiber clutches that's a problem in itself.
Mike