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Old 05-22-2023, 03:39 PM   #1
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Default 12 bolt pinion shim

To all, anyone have a good guess at a pinion shim for an oem 3.31 gear in a 12 bolt? There’s nothing scribed on the head of the pinion or ring gear. I could be wrong, but I feel like there had to be a quick reference when gm was setting up hundreds of rear ends a year back in the day. Any help is much appreciated. (NOTE: I did find documentation that said 12 bolts got a .030 shim).
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Old 05-22-2023, 04:27 PM   #2
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Default Re: 12 bolt pinion shim

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To all, anyone have a good guess at a pinion shim for an oem 3.31 gear in a 12 bolt? There’s nothing scribed on the head of the pinion or ring gear. I could be wrong, but I feel like there had to be a quick reference when gm was setting up hundreds of rear ends a year back in the day. Any help is much appreciated. (NOTE: I did find documentation that said 12 bolts got a .030 shim).
Yes start with .030 shim, set backlash, check the pattern
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Old 05-22-2023, 04:31 PM   #3
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Default Re: 12 bolt pinion shim

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Yes start with .030 shim, set backlash, check the pattern
Will do. Thanks Glenn
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Old 05-24-2023, 09:53 AM   #4
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Default Re: 12 bolt pinion shim

Back in the 60.s I did lots of GM muscle cars with 12 bolts & typical shims from .028 to .032 ..start with the .28 & old trick to prevent bearing from getting wrecked on & off many times to change shim is to heat the bearing up to expand so it slips on & off off easy
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Old 05-28-2023, 08:58 AM   #5
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Default Re: 12 bolt pinion shim

I use the reference when I'm having a problem setting up a ring and pinion or I do not have access to the original shim or the old gear was not set up correctly.

GM .035

FORD 8"-9" *020

FORD 8.8"-7.5" .030

ALL DANA'S .035

8 3/4 X 1 3/4 PIN .090

8 3/4 X 1 7/8 PIN .020

MOPAR 9 1/4 .020
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Old 05-28-2023, 09:43 PM   #6
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Default Re: 12 bolt pinion shim

12 bolts usually take the following, in my experience:

2, 3 ratios...mid to high .020's
4, 5, 6 ratios...low to mid .030's
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Old 05-30-2023, 01:17 PM   #7
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Default Re: 12 bolt pinion shim

.026 to .028 will get you close. Remember when setting a used gear set that it is highly possible that you will not see a good pattern. you can get close adjust backlash a little on the loose side monitor noise on coast adjust if necessary.
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Old 05-30-2023, 05:54 PM   #8
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Default Re: 12 bolt pinion shim

Doing a new set, .030-.032 is my default. On a used set, I'll hedge thinner by a couple thou. and pay more attention to the coast side. A bit of red Loctite on the pinion shaft where the front bearing registers and eliminate the crush sleeve.
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Old 05-31-2023, 09:04 PM   #9
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Default Re: 12 bolt pinion shim

Thanks everyone. We’re on the right track for sure. I set it up with a .030 and based on the master housing depth of 4.670, it’s dead nuts on (pinion depth of 2.755 plus the thickness of the pinion head 1.915 equals 4.670).
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Old 06-01-2023, 06:46 AM   #10
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Default Re: 12 bolt pinion shim

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Thanks everyone. We’re on the right track for sure. I set it up with a .030 and based on the master housing depth of 4.670, it’s dead nuts on (pinion depth of 2.755 plus the thickness of the pinion head 1.915 equals 4.670).
That's fine but pattern and backlash are what matter.
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