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Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Transmission - Rearend> How to identify Oldsmobile 12 bolt?
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Topic Review (Newest First)
12-13-2003 10:59 AM
elukas 425hp olds,

Could I email you a pic of the rear cover? I think it's the oddly round one you're talking about.

My email addy is If you shoot me a quick hello, I'll reply back with the photo, and I'd appreciate it a lot.



P.S. Sounds like this oddball is just not the way to go unless you want numbers matching!
12-12-2003 08:27 PM
Grusome Goat I had one of the 12 Olds rears several years ago. One of the main differences I noticed is the Olds rear axles bolt in, just like the B-O-P 8.2 10 bolt whereas the 12 bolt Chevy, 10 bolt Chevy and corporate 8.5 axles are held in with c-clips. The Olds rearend parts are very hard to come by and very expensive, I once called on a set of 3.90 gears and the price was over $450 and this was for used parts. I pulled the rearend back out and put the stock unit back in my GTO.
12-12-2003 01:06 PM
425hpOlds Unfortunately, I have this 'mystery' rear end in my 69 Cutlass. Here is what I know about it:

It IS a 12-bolt case with a 10-bolt carrier that measures 8.5", not 8.2". It was only in Cutlasses from 1968-1970. It is Oldsmobile's version of the Chevy 12-bolt rear end. It has an odd-ball bolt pattern on the cover, and the cover is almost completely round. The really strange part is that the pinion is only 12 splines, where as Chevy used a 28 spline for the 8.5".

It's main weakness is actually not the 10-bolt carrier. The weakness lies in the bearings. The bearings are smaller than the 12-bolt Chevy, leading to excessive heat and premature failure.

The main factor is the parts for the 'O' axle are 2.5-3 times the price of a standard 8.5" 10-bolt.

I am currently wrestling with the decision as to whether to retain the stock axle in a numbers-matching car, or to switch to a GM corporate 8.5" rear out of a 71-72 Cutlass. Either way, I'm upgrading the gearset to 3.42.

Hope this was helpful.
12-08-2003 02:49 PM
Stitch Yeah you're right Erik . I had a quick look at those websites and it seems that the 'O' type axle would be a bad choice , being weaker and with limited cwp availability . It's annoying that neither web site tells you how to identify it from the exterior , unless the only positive way is to pull the cover and either measure the ring gear or count the axle splines . Are you putting it in a Cutlass/442 ? . I guess the favourite thing to do would be to find a Chevelle 12 bolt but that's no easy task .
12-08-2003 02:30 PM
elukas "12 bolt rear cover = 12 bolt rear"

It's definitely a 12 bolt cover, but it's like there's this possible oldsmobile exception

"Wouldn't it just be a generic GM 10 or 12 bolt if it was from that period ? , or were there differences between BOP and Chevy ?"

Actually there was a difference between olds and the rest apparently on some of the cars produced during those years.

Here's all the details I could find guys:

Check it out and tell me what you make of it. It's like "c'mon a 12 bolt cover with 28 spline axles and a 8.2" ring gear!?!?!? WTF were you engineering guys thinking!?!?"

I wouldn't worry about it if I didn't need the strength of a 12 bolt.

12-08-2003 02:26 PM
Stitch Wouldn't it just be a generic GM 10 or 12 bolt if it was from that period ? , or were there differences between BOP and Chevy ?

Your not thinking of the 58 - 64 Olds rear are you ?

"All the olds cars built in america are supposed to have had this strange weak o type 12 bolt that's more like a 10 bolt strength wise. "

What about the 442 W-30 and the full size cars with a 455 , surely they would've had a full strength 12 bolt rear ? .

dang ponch I just noticed we registered at the same time but you got over 5 times as many posts as me

Here's a link from an Oldsmobile website giving 12 bolt tips , it only refers to it as GM 12 bolt and doesn't seem to differentiate between BOP or Chevy............
12-08-2003 01:58 PM
adb2f I have heard that there is a 10 bolt olds and I think there is a 12 bolt as well. As far as a 12 bolt chevy from the factory, I do not have that one. Ask the guy to send a pic of the diff with the cover off and maybe you could post it so somebody could tell you for sure.
12-08-2003 01:10 PM
poncho62 Never heard that one before. and I live in Ontario.

All I know is 12 bolt rear cover = 12 bolt rear.
12-08-2003 09:51 AM
How to identify Oldsmobile 12 bolt?

I know the ring gear of a chevy 12 bolt is 8.8" and the oldsmobile o-type 10 bolt carrier is supposed to have an 8.2" ring gear...

BUT... is there any way externally to tell the difference between the 2 rear ends?

What I have here is a rear end on a 69 cutlass a guy is selling.

He's sent me pics and thinks its a chevy 12 bolt. Yes, the case has 12 bolts, but if I know my oldsmobile lore only olds cars built in the oshawa, ontario plant had chevy 12 bolts between 1968-1970. All the olds cars built in america are supposed to have had this strange weak o type 12 bolt that's more like a 10 bolt strength wise.

So can anyone help me out?



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