Hi! Happy to be new here. Does anyone have info on the 8 3/4" GM "10 bolt?" (It has 12 bolts on the ring, 10 on the cover...) It's an oddball. I'm running a '78 Cadillac Fleetwood rear end with 2.28 gears and a posi, and would love to swap to a numerically higher ratio of 3.43 or 3.55ish... there were 2.73 gears installed in tow package rears but that's not enough! Btw for anyone wondering or internet searching this it seems to me that every large cadillac (non Seville) from 77-79 runs this rear. Every one I've ever seen at the junkyards (about a million) runs this rear, and even the factory '78 Cadillac manual specifically states that "The rear axles for all models except Seville, have an 8-3/4" diameter ring gear." I've heard rumors that you all or Tom Parsons (DZ AUTO --no longer a member?) may have knowledge as to whether the GM 12 bolt 8.875" ring and pinions or other setup may fit this rear end? (the ring bolts to the carrier with 12 bolts, not sure of pinion shaft diameter or other inner workings.) I've also heard unverified rumor of 3.73 gear swaps and factory 3.42s in these rearends--any idea? I know this rearend is an oddball but it sure is stout, and if there's any info out there to step up the gears and rebuild the posi, it could be really handy. This rear is still falling out of the trees in 77-79 cadillacs and is a direct swap into 70's-80's B bodies and cars with the triangulated 4 link. Thank you so much for any info you may have! --Aaron (brokeviolinist)
seems to me if you want good gear selection, you would get an 8.5" rear out of a B body and use that.
I may very well go that way and have plans should I need to but this rear is considerably beefier than the 8.5 in many respects... and it's already set up on the car.
The only 10 bolt cover/12-bolt ring gear I had the misfortune to waste money on was pulled out of a mid to late-70's station wagon. -seems it was either a Cutl*** wagon or the Olds B-body wagon. I got it for the 3.08's and then discovered it wasn't an 8.5". -thought they were just an odd wagon-thing, don't know if this is the same rear you are talking about.
Here's more info https://www.ringpinion.com/b2c/partcats.aspx?SearchMode=Diff&TypeID=5&DiffID=220 Not a lot of parts. FWIW, the 8.5" b body 10 bolt takes lots of abuse. Unless you are doing clutch dumps on slicks, I've seen 10.5's go low 10's in 4000# Bbody's. Here's a friend's car running 5.6x gears in low 11's, another has run 10.29. Sent from my XT1585 using The H.A.M.B. mobile app
The strength of a GM 8.5" rear comes from the fact that the pinion bearings are bigger than what's in a 12 bolt Chevy rear.They came with 28 or 31 spline axles (like a 12 bolt). Pick up trucks came with 31 spline axles and leaf springs making them easily adaptable. Aftermarket posi units make them even stronger. The 12 bolt Chevy rear end is only marginally stronger than the 8.5" 10 bolt rear end because of it's 8.875" ring gear and bigger carrier bearings. Get big Ford housing ends, bearings, drum brakes and aftermarket axles for a nearly bullet proof rear end. Just get the axles and drums drilled for whatever bolt pattern your wheels are. There are people who have run just such a combination with a girdle cover and a billet steel drivers side carrier bearing cap into the eight second quarter mile zone.
Thanks for the info! RockAuto has a few things for the 8.75 as well (search '78 Cadillac Fleetwood) like bearing kits but it's a little difficult to sort through whether they've stocked it/labeled it correctly for this particular rear. Great picture!
Good to know--thanks for the info! I'll feel a little better about it if I end up going with the 8.5"... If I happen to find any further info on the 8.75 I'll be sure to post it here as a matter of public service. There are a couple guys I need to call...
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