# 12 bolt ID



## Ziggy 7096 (Apr 10, 2018)

I have bought a 1971 GTO 400 4 speed muncie with according to the math a 4:66.1 gear. Numbers off the ring gear are 9A42 GM ZP 169 2 it has a roughly 1/8 inch spacer behind the ring gear. The only numbers on the carrier are T67 and there fins cast on the ring gear side of the carrier. the passenger axle tube has SD stamped on it and nothing else. The center section has 130 on the passenger side and BH on the drivers side. Any help identifying this rear end would be greatly appreciated. I was told it may be a judge but we all know that story Ill wait for PHS to tell me what it is. thanks again in advance Bob Ok I figured out its an olds O rear end but I still have things that dont add up like it has c-clip axles is there some way these interchange ?


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## Pinion head (Jan 3, 2015)

All Type "O" Olds cutlass/442 10 bolt rears take bolt in axles/ the axle tubes do not neck down at the end like a c-lip axle tube. All Type "O" Olds ten bolts used a smooth rear cover with 12 bolts to attach to the housing. All type "O" Olds 10 bolts will have the large square lugs on the bottom edge of the rearend housing. Having ground these lugs down on multiple dozen of 8.5 housings, it's possible someone could have ground down the large lugs on a type "O" Olds 10 bolt.

Not exactly sure what you have. the 12 bolt rear cover does have the big splash lip on it? From your info, am not able to decypher the date on the ring & pinion . I've disassembed numerous '72 12 bolt rears that did not have typical style of dates inscribed on their original 3.07 & 3.31 GM ring and pinion. 

To ID what year usage of C-clip axle 12 bolt rear, you need to look at the lower webbing of the center housing, on the front, on the drivers side.

Following are the casting numbers of the early 12 bolt A-body (Malibu-Chenille-Elco ) housings. Most of these early factory assembled 12 bolt rears went under 327 base 4bbl powered Elco's and Malibu's. Chevrolet sourced the 12 bolt rear when its cars were equipped with 275 hp and more powerful engines. Nearly all '65 & '66 "chevelle" 12 bolts that I've sourced have had 3.31 GM gears, most have been single tracks.

Casting number on center housing:

3850140......'65 "chevelle"
3875745N...'66 "chevelle"
3894939......'67 "chevelle"

'68 & later c-clip A-body 12 bolts* *note #*
3917124.....'68 "chevelle"
3917124.....early '69 "chevelle"
3959038.....late '69 "chevelle"

1970-1972 usage:
3969278.......any of these latter "278" castings can be the 12 bolt A-body housing casting number in a '70-72 Chevy "A-body".
3969278N....
3969278NF...

'70-72 Pontiac A & GP's with factory 455 engines, as well as '70 GP's factory built with 400 4speeds received a McKinnon built 12 bolt which was built with its own style cast "278" center housing. McKinnon cast 12 bolt center housings did not have their cast date in the rh lower rear webbing. All other 12 bolt housings built out of the other three 12 bolt c-clip car axle plants had a very legible date in the lower rear passenger side webbing. 

*# for '68-69 models,*
Canadian (Oshawa plant) built Olds & Buick A-body's were equipped with a McKinnon Industries (Canadian plant) built 12 bolt rear. Oshawa built '68 & 69 Olds & Buicks as well as '70 Oshawa Buicks were not built in huge quantitys. As most of these low hp mundane transportation type A-body's were sold well up north, most have been parted & are now history. Be advised, the '68 & 69 Oshawa built Olds & Buick McKinnon 12bolt rears will often have a stamped two letter ratio ID code beginning with an L. 2.56, 2.73, 3.08, & 3.31 are the typical gear ratios. Some McKinnon 12 bolts also made it into '70 Buick Skylarks built out of the Framingham plant instead of '70 BUICK 8.2 10 bolts, have pulled half a dozen, all were 2.73 single tracks.

Feel free and post some good pics, if it went in a muscle era GM, I've had my hands on one.


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## Ziggy 7096 (Apr 10, 2018)

It has a smooth cover 12 bolts, 10 bolts in the ring gear and it look like someone ground all identifying numbers off. I can see spots where its been ground on, like the axle tube on the front passenger side and the same on the carrier housing almost all the numbers have been ground off except for the two I mentioned in the first post. The same with the carrier itself no numbers except T67, but i can see grind marks on the end of the carrier and it has clutch packs if that helps. I'm thinking its going to be easier to put a chevelle 12 bolt in it
Thanks again


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