HELP GM 8.5 axle rebuild... Will this run?

Setting up these gears is a science that few actually can do. Are you experienced at this? Did you read the FSM for the process and tools needed? I think you should start at the beginning and post your results.

Did you use all new bearings? A new crush collar? What pinion gear shim thickness, and the total pre-load you used on it. Did you use a pinion depth setting tool?

FluoFerret said:

0,041mm shim-setup

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That is a mistake, yes?

Did you use new carrier side bearings? What kind of carrier shims did you use? A kit of multiple sizes? Did you find zero lash and add the correct pre-load shims?

After all that is perfect, only then should you be looking at gear backlash and tooth contact pattern, adjusting as needed.

If you are experienced at this, I apologize, but you did not give enough info to draw any conclusions.

Maybe it's my eyesight, but it APPEARS to me the drive and coast sides are a bit leaning toward the toe. The pinion depth looks pretty good to me, but backlash might need to be increased a hair. Again, to me, it appears ALMOST there. I'm not an expert, though.

That's all I got. Out of all of them, the coast side pattern is the least worrysome. In other words, if the drive side is set fairly centered, that's the most important part of the equation.

Ask an expert. A guy named Jim goes by the username "monzaz" on a bunch of car forum sites. Don't recall his last name. He is someone I consider a valid expert on ring and pinion stuff. He's only done about 10,000 rear ends. And didn't have to go to prison to do it. 🙂

They got a forum there. Contact him there and if he decides to help you, which he most likely will, he can just look at what you got and tell you what's what. Yeah, he's THAT good.


To join the forum there, or just read up on stuff (you're not the only one that's had questions):

If you got all the parts, aint no shame in taking it down to someone with a shop and a bit more experience. Plus if it blows up, you know where they work.

MrSony said:

If you got all the parts, aint no shame in taking it down to someone with a shop and a bit more experience. Plus if it blows up, you know where they work.

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I think the issue with that is he is in Luxembourg not the US

OK, I get it. You are forced to attempt it because, well, you are the last resort. No problem. I used to do it with just a set of feeler gauges. First off, there is a wonderful explanation of all this in the Factory Service Manual. You really need to read up on this or you may make some very expensive errors. Is that a 1980 Pontiac? Nice. Any FSM for a G or F body from that era will have the 8.5 rear differential section. They are all set up pretty much the same. I would take the carrier out and double check the pinion pre-load just to be sure.

You must first understand the concept. The pinion and carrier are held in suspension by having an amount of pre-load on them. That way they will stay in alignment under the torque from the engine. If they move, they lose the relationship of the pinion and ring gears, then they wear, or break. So you first assemble the pinion gear and bearings, check the pinion depth, set the pre-load, and then leave it alone. TIP: I have always checked the pinon depth with a factory depth tool. It has never been off from the original pinion shim by more than a thousandths of an inch or two. Thus, we usually re-use the original shim. Also, check the pre-load without the carrier in place. You just want the pre-load on the pinion gear. The reason they used a crush collar is that as you tighten the pinion nut, it crushes the collar. Once you get the correct pre-load, checked with an inch pounds torque wrench, you stop turning the nut and the collar is now the exact size to maintain that pre-load. The reason that a shim might be used is in racing, when they take rears apart and put them back together and speed is essential. The shim is only good if the pre-load was first set with a collar, removed, measured, and a shim the exact same size put in. Otherwise, how can you know what shim to use?

So, please find a manual, read it, and go back to the pinion setting. Ebay has them as well as factorymanuals.com. Worst case I find one and ship it to you.

.005 backlash is too tight. No less than .008" is recommended and if it were .012-.013 it will work just fine. I target .008-.010. Think of backlash like crank bearings - too tight and everyone knows, a little loose and no one knows.

When you're measuring rotational torque -be sure to not have the pinion seal installed.

When installing the carrier shims, if you can push them in with your fingers, then they are loose. Add a couple of thousands to each side until they can be tapped in (not hammered in, but tapped in) with a small hammer. I put the diff in, with the pinion not installed and figure out how much total shim I need for both sides combined, then record that number. When you're all done, be sure the carrier shims equal that number or .002-.003 greater.

So looking at your pattern knowing the backlash is too tight and carrier shims are too loose is somewhat of a waste of time in my opinion.

565bbchevy said:

I think the issue with that is he is in Luxembourg not the US

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Germany doesnt have a publicly available drivetrain service shop?