8 3/4" sure-grip swap and rebuild questions

[MopaR&D]

Why would you open it up instead of closing it up to 7 or 8? It's going to wear past 10 after you drive it a while.
Aren't tighter clearances quieter?

I opened it up because the instructions that came with them from Doctor Diff spec'd .010-.012" backlash. If you look back to my previous posts I first set it up at .007" which is what the instructions from Motive said (.007-.009") so after the 500 miles of break-in it opened up to .009" and the noise didn't get any better or worse. Someone correct me if I'm wrong but the backlash itself doesn't affect noise, only the location that setting places the teeth in contact at. When under drive or coast the ring and pinion teeth are in constant contact.

 

[Jadaharabi]

Backlash is how deep the pinion is sitting into the ring gear. So it most certainly can affect any gear whine.
I may be a dipstick and not know much from only been around for a while but I have found the looser the backlash is the more likely you are to have a noise. And not to forget the clunk when you back off the throttle.
Personally I think it's the brand of Gears that you got there they are famous for making noise. But when you're running open headers you don't really notice it.

 

[jpl572103]

I just disassembled my first 8 3/4 housing with 3.55 suregrip today. Very heavy. I stumbled upon this chat and wanted to give a nickles worth of in one ear and out the other. Having done plenty of GM stuff since the 80's. In all my rebuilds of reusing the case with a new ring and pinion, I would always reuse the shim from the old pinion gear found under the old pinion bearing. If the shim was damaged and couldn't read it or the case was damaged and was using a new case(housing), I would need to use the pinion depth tool to determine what thickness shim to use under the new pinion bearing.

Does 8 3/4 use a shim under the pinion bearing? If it does, that could be the whole reason this young man is dealing with a bad contact pattern. Find that original shim that was installed under the bearing attached to the original pinion gear that was in this housing. Use that shim under your new pinion bearing that's going on your Motive pinion gear.

Leave your ring gear out for the next part. I always crush my sleeve with a 1/2 " impact. It's the way I was taught. The only time a 1/2'' impact would not crush a sleeve for me was when I did a GM 10.5. I had no choice but to use the J-tools that would hold the pinion yoke while I used a 3/4 breaker bar to tighten the nut to crush the huge sleeve(real *****). But that gear had spanner nuts too like the 8 3/4. So once the pinion was set, you were home free.

So on your 8 3/4 after you get your shim figured out, tighten the nut a little at a time(bop bop bop) and then rotate the yoke by hand a few times. If it spins freely, bop it some more. As soon as you start to feel resistance when you rotate by hand, get your inch lb torque wrench and see how many inch lbs it takes to rotate that yoke. Keep rotating while your crushing. Should rotate smooth but gradually get "resistance" Keep (bop bop bop) just a little at a time until your pinion preload is 25-35 inlbs on new pinion bearings and 15-25 on used. If you tighten too much, you will have to start over. Not a big deal.
It takes trial and error to be able to "feel" and not use the tool, you should just use the inch lb tool. If using the old shim, the old case, new gear, the pinion depth will be perfect after setting preload. Of course this also depends on your race and bearing install and the manufacturer of the ring gear. Hopefully specs are the same as OE. Anyways, Once your pinion preload is correct. Time for the ring gear(crown) install.

Next install your ring gear. 8 3/4 is easy. Spanner nuts!!!

Set your lash to .0007 for new gears. You have 500miles. I would set it for .0007