Question :Un Différentiel 12 Bolt de olds.. Vs Chevelle

Question : Help! Again.

I need new rear axle bearings but can't ID the axle to get parts.

It's a 12 bolt posi. Apparently GM since it's in my Chevelle, But the diff. cover is round, not offset like Chevy 12 bolts. I've pulled the wheels and there is a cover that bolts over the bearings. I've been told, sight unseen, that it may be from a Cutlass or Skylark?!?!
The axle code is CFD3.

Can anyone tell me ANYTHING about this axle, or where I might find the decode? I've looked all over the net.
Any info is much more than what I have now. Thanks!

Réponse : It is an oldsmobile 12 bolt, which is really a 10 bolt (10 bolts holding the ring gear to the carrier, which is really what 10/12 bolt refers to). I normally hear it referred to a type "O" rear with a 12 bolt cover.

N-bolt Differences

An Olds 12 bolt rear end is not a "true" 12-bolt. It may have 12 bolts on the outside cover, but it is a 10 bolt gear on the inside. The more common type "C" is very much different than the type "O".

A "true 12-bolt" has come to mean a Chevy c-clip 12-bolt rear. Oldsmobile 12 bolt rear ends differ from Chevys internally. In other words, the guts of a Chevy rear end will not fit in an Olds rear end housing, and vice versa. The phrase, "more like a 10-bolt", is usually talking about the size of the ring gear and/or strength of the carrier. The guts of a 10 bolt would not work or fit in a 12 bolt, and vice versa. There are differences between the 10 bolt and the 12 bolt both internally and externally.

Note also that there are two different ring gear carriers, one for 3-series ratio gears and one for 4-series gears. 3 series and 4 series carriers are for the 12 bolt Chevys. The difference is the dimension from the ring gear mounting flange to the pinion centerline, due to the larger diameter pinion required for the lower numerical gear ratios. A set of 4-series gears can be installed in the 3-series carrier with a spacer and longer bolts (not desireable due to reduced strength), but the reverse is not true.

The ring gear size is the size of the largest gear, parallel to the wheels. A 12 bolt Olds has a 10 bolt carrier that measures 8.3", whereas a 12 bolt Chevy has a 12 bolt carrier that measures 8.8". A 10 bolt Olds out of a 1971 has a 10 bolt that measures 8.5". 1968 and 1969 10 bolts might have an 8.2" ring gear. Starting in 1970, 10 bolts might be 8.5". Newer 10 bolts found under the last RWD Cutlass and Chevy are little 7.5".

Oldsmobile bolted the axles from the inside of the housing, whereas Chevy used c-clips. As for Buick and Pontiac, they used different types of posi actuating items called "cones" and "spools", whereas 12 bolt Chevys and Olds used what are called "clutch packs".

Type "O" Oldsmobile Rear Ends

The biggest weak spots in Olds rears are the wheel bearings and the availability of parts. Positive axle retention is good, but put in the new "updated" style wheel bearings for more strength.

Gears for the Olds 12 bolt rear are hard to find. The "O" was only manufactured for 2-3 years. So there isn't enough call for someone to start casting and machining new cases. While posi cases are not available (as far as I know) rebuild kits are. Unless the case you have is outright cracked or otherwise junk you should be able to rebuild it.

Type "C" Chevy Rear Ends

This rear end retains the axle shafts with "C" clips. These do break from time to time, and the result is the axle shaft works its way out of the axle tube. A hint that a clip might be broken is a faint moan from the rear axle when backing up (mine made no noise going forward). Moral of the story: stick with your Olds rear.

The "C" rear can be modified with "a C-clip eliminator kit". That solves that problem of relatively fragile broken "C" clips.

If you want/need a stronger more available rear, the "C" 12 bolt, will bolt in with no changes to your car. It came in the C*evelle after all. The rear U-joint will need to be changed to a hybrid one. NAPA and others carry such a U-joint.

Given the "C" 12-bolt is almost a 9" ring gear, parts are as common as sand on the beach, and almost as cheap. It has some redeeming values once modified for performance. NHRA won't allow the faster cars to run one without the eliminator.