12 bolt pinion shim

[3 window]

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).

[Glenn Briglio]

Originally Posted by 3 window

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

[3 window]

Originally Posted by Glenn Briglio

Yes start with .030 shim, set backlash, check the pattern

Will do. Thanks Glenn

[prostockpete1]

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

[Chevy55]

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

[Mark Yacavone]

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