My rear needs all new bearings. Is it safe to assume that the backlash will still be correct when just using the same shims with the new bearings. Or, should I plan on having to go through the whole process of resetting the backlash? I plan on checking the contact pattern either way, but Im curious as to how much it changes with age. The gearset looks ok and doesnt show any pitting, but all the bearing races are severely pitted from 300+ thousand miles of use.

 

You should plan on setting everything up again. The last bearing change that I did required a .003 change in the pinion shim to get the pinion depth right again. This was in a 2006 GTO, so there were no differential shims to change. Never assume that a replacement bearing will be the same overall thickness as the one that you are removing. Also, use good bearings. Don't use anything made in China.

 

Thinking back on it, the backlash was a bit excessive, so the whole thing probably does need to be reset, or at least checked.

As far as the proper pinon depth, is the only way to check that essentially by the contact pattern on the gears?

 

I don't believe I have EVER taken apart a 8.5" or 7.5" 10-bolt and found ANYTHING BIT a .035" shim, or had to use ANYTHING BUT a .035" shim in one I was building. Of course, the only gears I have ever used have been the higher-quality ones; Dana, Precision, Superior, US Gear, AAM/Motive. Evidently GM managed to get the machining of that one housing feature fairly consistent.

The stock carrier shims are cast iron. Frequently, excessive backlash is caused directly by them; they wear if anything spins in there, which it usually does, eventually. It is rarely a good idea to re-use them.

Remember, you're shimming THE HOUSING to match THE GEARS, not the reverse. Gears, bearings, and carriers are MUCH more precise than the housing; probably 20 times tighter tolerances. If you imagine what you're doing as holding a "perfect" set of moving parts out in space somewhere, and then choosing shims to "hang" a randomly variable housing around them, you'll be well on your way to UNDERSTANDING what you're doing, instead of going step 1 is .... step 2 is ... step 3 is... , blindly, all the way through without knowing WHY, and hoping for the best at the end.

A pinion depth tool is another good way to check that. But as said, I've NEVER had to use ANYTHING BUT a .035" shim, and I've NEVER had one that wasn't satisfactory. I used to actually check it, with honed-out head and tail bearings so I could easily disassemble them; but after doing however many and never having to change the shim, I quit bothering. I don't even know for sure I could put my hand on those bearings any more, I haven't used them in so long.

Carrier shims are a different matter. Those vary CONSIDERABLY from one rear to another. But those are EASY; you just use them to set the backlash. Backlash is like piston ring gap: a tiny bit too much doesn't hurt anything and is not really noticeable, but a tiny bit too little means INSTANT DESTRUCTION. Remember, the gears get PRETTY HOT during operation, and they "grow" a little; therefore most of the backlash disappears during use. If there's not enough, the gears try to occupy the same volume, and as we all know, that's a bad thing with metal parts. They don't like that. Therefore I tend to shoot for the high side of the spec, around .010" - .012" in a 10-bolt.

I also like to use a crush sleeve spacer, so I can tighten the snot out of the pinion nut. The way the spacer works is similar to your front wheel bearings: think about how you tighten the nut on those, and as you tighten it, the play goes away, eventually you reach a point of zero play, and as you keep tightening, you start to require torque to turn the rotor; and you set the nut to where that torque is correct, and lock it in place with a cotter pin. The pinion works the same way, except that a coter pin isn't enough to hold the nut. Instead, the crush sleeve provides something to tighten the nut against; and you have to stop tghtening when the preload on the bearings is right, as measured by the torque required to turn the pinion. But with the spacer, you select the right thickness of that to yeild the right distance between the bearings to produce the correct preload on them, and then you can tighten the nut until it screams for mercy. I HIGHLY recommend using that instead of a crush sleeve.

 

Yes, you set the pinion depth by reading the contact pattern and making adjustments by what the pattern tells you. This is usually very difficult with used gears due to wear on the gears. I almost always use the coast side of the teeth to set the pinion depth with used gears because that side of the teeth get a lot less wear on them and the pattern is usually much easier to read. If you can't read the pattern then just stick with the original pinion shim.

 

The stock shims, at least the ones for the carrier, where unworn and the carrier was still nice and snug when I went to pull it. Im not really particular to whether they are new or used, but they essentially looked new when I took them out.

The backlash did seem kind of loose, but the final verdict will be when I measure it with the carrier re-installed. The pinon bearings are shot as well, so it could be the result of that as well. I dont think its gotten to the point where the races have spun in the case, but any farther and the rear wouldve nuked itself.

Im familiar with why the rear needs to be shimmed as the housing is a large cast iron assembly that cant be machined nearly as precise as a set of gears or carrier, and the hypoid gears themselves have a very specific requirement for the mesh. I just want to make sure Im not overlooking somthing during the setup.

I was thinking of using a spacer instead of the sleeve for the re-assembly, but as far as teh correct thickness, I assume that they come with shim packs that allow you to set the desired thickness for the bolt torque and bearing preload? That seems to be a lot easier to deal with than the crush sleeve. Are the 9-bolt crush sleeves teh same as the 10-bolt ones?

 

As far as the carrier shims, can they usually be tapped in with a drift w/o damaging them? It did seem to take a little persuasion to get the carrier out of the rear as it was a snug fit. I can build a case spreader if it comes to that, but if a drift will do the job just as well, id rather use that then do a whole lot of welding to make something elaborate.

As for the fitment of the carrier, the shims also set the preload on the carrier bearings. Is there anything I should watch out for there? Or will just selecting similar shims and installing be good enough?

Im familiar with how to service a rear, but Ive never bothered to actually do it before. In the past, I usually just chucked them when they went bad, and snaged another one from a yard. This one is still in good shape, so Id like to refresh it instead.

 

This is what I use to install the shims. It's just a little tool that I made up from some scrap metal. The shims need to be tight enough that you can't pull the differential out by hand, but not so tight that you damage the houisng or shims when installing them.

 

There needs to be quite a bit of preload on the carrier bearings. Rule of thumbs is, set the backlash with as much shim as you can cram in there; then when assembling it, add at least .005" TO BOTH SIDES EQUALLY. Use the thin shims in between 2 of the thickest ones they supply, so that the thin ones don't get involved in the cram-in process.

I usually use 2 big C-clamps on the bearing races, to compress them. I don't have a "case spreader". I've never really had any trouble getting em REAL TIGHT that way. If there's not enough preload, then the backlash won't stay consistent, and it will tend to be noisy in the "float" condition.

Your description of how the crush sleeve eliminator works is accurate. You typically assemble it with all of them in there, then measure the end play, then take that much shim out; then take out the smallest increments you can until the preload reaches the desired value. I prefer to be at the high end of the spec range; or even, a little too much preload (the operative concept being A LITTLE, like, no more than 6 or 8 in-lbs above spec) rather than too little, if that choice has to be made. Lube the bearings with white lithium for the test.