I'm pretty sure this has been discussed many times before but it's hard to find out the right answer or what to look for. Like many(all) I want to upgrade my diff but am limited due to the Olds O type rear end. I have a 72 Cutlass S that had the 2.73 gears in it. I wanted to upgrade to 3.73 gears and a Detroit Truetrac but was informed that they won't fit and I would probably need a custom diff. I started looking on Craigslist for a Chevelle rear end but they aren't cheap. A guy had an A Body rear end that he later found out was a 12 bolt cover, O Type. I basically just need the housing since I would rebuild it, but how can you determine a Chevelle rear end from a Cutlass rear end? Can you tell but stamping or casting numbers? Any info or links to comparison images would help a lot.

 

If you have the original rear end its not a type O. The 1971-72 Cutlass used the corporate 8.5" rear end. Those are easy to find posi carriers for and has a great selection of gears available. If your not sure get a picture of the inspection cover and the code stamped in the passengerside tube and post them here.

John

 

Here is a picture of a 71 cutlass rear end. If yours looks like that, 10 bolt cover, those little "ears" on the bottom of the differential (on top in the picture), than it's a corporate 8.5" with 28 spline bolt-in axles.
You can PM monzaz (jim) here on CO. He can help you identify it and set you up with good quality parts, whether it's a type O, corporate, or any other. Here is a link to his website He is a good guy to deal with
http://www.jdrace.com/



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My original 1971 10 bolt rear has a big "O" on the forward facing part of each center section axle tube receptacle (where the axle tubes fit into the center section).
That would be an easy way to ID an original 8.5" 10 bolt.

 

Do you mean the difference between a Type O and a Chevy 12-bolt? If so, for quick ID purposes, the Chevy will have an S-shaped baffle in the inspection cover that is a dead giveaway. Internally they are completely different. For starters, the Type O only uses 10 bolts to secure its 8.5 inch ring gear, whereas the Chevy uses 12 for its 8 7/8 inch gear. Also, the Chevy uses C-clips to retain the axles and the Olds bolts them from the inside of the housing.


Etc.

 

Are you referring to this?

 

Yup.

 

Yeah 70cutty, that's what mine looks like. It's a 8.5" gear and 28 spline bolt in axles. So what does a corporate diff actually mean? Any GM 8.5" diff in a parts catalog will work? 20150309_173103.jpg
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20150310_204754.jpg

 

Yes, you just have to ensure you get the differential that matches the gear ratio (2.73 and up is one series; 2.56, 2.41, and anything else lower than 2.73 is another).

 

The corporate 8.5" rear ends were used in lots of different GM cars and even 1/2 ton pickups/vans. I'm not sure, but think these were used from about 1971 through 1990ish. So you could get a posi carrier out of any one of these other cars and have it installed in your housing.

As mentioned by 70cutty monzaz is one of the members here and he sells parts or complete rear ends. Jims a great guy to do business with.

John