12 bolt ring gear 10 bolt cover/ 12 bolt chevy

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12 bolt ring gear 10 bolt cover/ 12 bolt chevy

Post by 652plus2 » Mon Aug 27, 2018 2:24 pm

Can you explain the fundamental differences between a 12 bolt 1965 Pontiac ring and pinion and a Chevy 12 bolt ring and pinion? Can Chevy gears be machined to work in a Pontiac 12 bolt ring gear /10 bolt cover rear?

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Re: 12 bolt ring gear 10 bolt cover/ 12 bolt chevy

Post by monzaz » Tue Aug 28, 2018 12:47 am

1965-1970 Pontiac (FULL SIZE CAR REAR) That is your main difference. The rings are the same 8.875 diameter But that is where the similarities STOP.
Ring gear bolts Pontiac are 7/16" x20 (Right and left hand thread early or late)
Ring gear bolt s Chevy are 3/8 x 24 (right hand thread only)
Pinion shaft on the Pontiac is 1.875
Pinion shaft on Chevy is 1.625
----- -- THIS is the main reason your not going to fit the Chevy gear in a Pontiac housing.
Pinion spline Pontiac is 28
pinion spline in the Chevy is 30
Pontiac rear is bolt in axle 31 spline and the Chevy is 30 spline c-clip axle

In 1971 -1976 The Pontiac rear was used in all the mid HP engine from 400- 350 cube motor with 4 bbl motors Gears will interchange from the 2 eras BUT you MUST use the yokes from each era with the era gears. 1971 -1976 era pinion spline is 30 not 28 like the 65-70 era. The gears will interchange
Axles will again NOT interchange because of the axle bearing differences and also the spline pitch was different too.

1977 -1985 there was a final form of this Pontiac rear and they really down sized it ...remember the cars in this era were way under powered so really the 10 bolt 8.5 would do the job just fine . pinion shaft decrease from 1.875 to 1.625 (basically the same 8.5 pinion was used in the 8.75 10 bot cover with 12 bolt ring design . It was used in the Caddy DeVille and wagons and maybe special tow package ordered cars of the era. Again NO PARTS in the aftermarket for this 8.75 rear.

Hope this help cover the Pontiac FULL SIZE car rear for you. and why you can not interchange the gear sets. Pinion shaft size is too small to transplant into the Pontiac housing - There could also be a different pinion to ring engagement height also But we never really went that far to check... Just not a BIG following for high performance ratios for the rear these days...Most cars have been demo derbied in the past. GM made steep gears for these rears and you can from time to time find a 3.73 3.90 4.33 gear set at a meet some one is selling that they have no idea what it fits we found (2) 4.33 gears that way 7-10 years ago. Sellers though they were truck gears...lol.

Good luck on the hunt.
Jim Mitschke
J D Race
J D Race

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