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GM 8.5" gear pattern

9635 Views 9 Replies 7 Participants Last post by  55_327
Swapping in 4.10's to my 8.5" 10 bolt. I am now in the process of setting up the gear pattern and shimming the pinion/carrier. I have a setup Pinion bearing so it is easy for me to pull it apart and change shims. I just needed a baseline for somewhere to go from to get things started. There were no small carrier shims when I took it apart, just the 2 very thick pieces that fit snug behind the races. I placed those back in and they are very snug again (have to tap them in). Anyways take a look at the pics and let me know what needs to be done to correct the pattern from here. I was able to get a fairly clear pattern. I know it is too far towards the toe right now, but at the same time is centered between the root and top of the tooth. What would you recommend I do to move the pattern towards the heel? Remove pinion shims or increase backlash? This is with 0.050" pinion shims. Thanks in advance 馃檪
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Way too much pinion shim.

Go down to around .030-.032" and try again.

It helps if you know what the factory pinion shim thickness was for the gear set you took gives you a starting place for the new set.

People think of gear set up as shimming the gearset to the housing....when the reality is the gearset is made with a tighter tolerance than the housing is, what you are really doing is "backfitting" the housing to the gearset. That is why the original pinion shim from the original gearset in that housing is most likely to be within about .003"卤 of being correct. And it is typically around the .030" thickness range....I've never seen one need below .024" nor above .036".....not to say that there might be some hpoousings that need wider out there, but it can't be very common, I've been in more than 2 dozen 8.5"s and not seen a wide shimmed set yet. Same for 12 bolt, 7.5", 8.2" and even 8.8" Ford or Dana 60
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scroll down for an easy to follow chart.
take 0.010" out of the pinion and try again.

Make sure to keep the backlash about 0.008" or so.
Thanks for the replies and info guys, much appreciated.

Went ahead and reshimmed the pinion down to 0.035". The original shim behind the stock pinion looks roughly to be around 0.035" as close as I can tell by comparing shims beside it (pinion still pressed on original pinion). My shims are in 0.005" increments so can only adjust by that much at a time. Take a look at the new pattern. It definitely looks a lot better and closer to the heel than my first pattern at 0.050". The drive side is still closer to the toe which is desirable for high performance use from what I have read, which the car will be seeing. The coast looks pretty damn centered. Both sides are closer to the top of the tooth compared to the root which maybe I could center a bit more by adjusting the backlash? The backlash is measuring 0.008"-0.009" all around so I don't really want to mess with it if I have it close enough right now. I would only be reshimmimg the carrier a few thou to maintain minimum backlash so I really don't see that minor of an adjustment bringing the contact down the tooth much. Anyways thanks again guys let me know what you think.

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The pinion is not deep enough. You need to add about .003 to .005 to the pinion shim. Adjust backlash again after you change the pinion shim. You can't fix this with backlash. This is a pinion depth problem. Don't worry a lot about toe to heel. Pay more attention to root to face. Right now your pattern is too much to the face and need to move to toward the root. Adding shim to the pinion will fix this.
I agree^^^, and it looks lke Imsport had the best handle on it whenhe suggested the .040" shim.(y) Looks like it is going to end up somewhere near that.

I find it somewhat curious that the OP only has shms in .005" increments....I didn't know anyone sold them that way.
I've never seen them that way either. I first thought that some of them may be stuck together.
If this is an aftermarket gear, the pinion depth should be etched on it. This is measured from the centerline of your axle. Hit that and the rest should go smoothly. I use a Pinion Depth Gauge made by Starrett that I bought thru Zoom gear in the early 80s. There is a preload on the carrier bearings that you need to find. There is a total thickness required for 20 inch pounds to move the installed carrier. Then you subtract from one side and add to the other to get your backlash.
If this is your first 10-bolt, make sure those pinion bearing races are fully seated. I thought I had my first one set up great until I drove the car. Ended up with a lot of clunking on the test drive, so tore it apart and found the problem! Had to reset pinion depth and of course pre-load. Fortunately I didn't damage anything.
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