David DeCan

I am about to install a true Chevy 12 bolt rear in my 70 Cutlass. Currently the car has a 350 CI engine and TH350 Transmission and an "O" type rear.

I will be swapping in a 455 using the 350 engine mounts and frame mounts as suggested here in Classic Olds.

For now I am going to leave the TH350 transmission in the car.

Question:

Driveline length will stay the same--as the TH350 remains in the car. What do I have to do to the driveline to connect it to the Chevy 12 bolt? U-joint change etc?

Thank you.

joe_padavano

The Chevy 12 bolt axle has a different dimension from the pinon flange to the axle centerline than does the Type O. This results in about a 1" difference in driveshaft length. If I remember correctly, the Chevy axle requires a longer driveshaft.

David DeCan

Having done this same swap years ago, I am remembering a conversion u-joint. Yes/No??

Run to Rund

Chevys generally used smaller u-joints, about 3.25" across while Olds used a stronger joint that was 3.62" across. There are flanges available for the 12 bolt taking the big u-joint, a better approach than a conversion joint. However, the installation of the flange must be carefully done to preserve the pinion bearing preload because of the crush sleeve between the bearings. I recommend installing c clip eliminators and stronger axles are a good idea too, when using the Chevy diff.

monzaz

The 12 bolt chevy is a direct swap with any other rear in any STOCK FACTORY A-Body (excluding 8.5 corporate 71 72 )

Meaning if it came from the factory with a 8.2 BOP or 8.2 chevy or 12 Bolt 'O' ALL you need to do is swap a U-joint NO Drive shaft mods.

372 Moog (U-joint) is you NEW drive line friend.

NOW if you have a 1971 1972 Original 8.5 Corporate rear you will have drive shaft length issues.... ANY swap with the CORP 8.5 will require drive shaft length mods.

So if you are swaping out the old 12 bolt OLDS rear (12 cover 10 bolt ring) All you will need is a u-joint at the most...IF EVERYTHING IS STOCK from the factory...THAT is usually the big IF after 45 years of being beat on, bought, sold, resold, restored, crashed, reborn, etc....lol.

So as long as you keep the 8.5 Corporate rear out of the mix all rears swap easy. Mix the 8.5 Corporate rear in any part of the mix you will have drive shaft length issues.

REMEMBER THIS IS IF THE CARS ARE COMPLETELY STOCK FROM THE FACTORY. I would say ALWAYS measure with rear in the car suspension loaded before cutting or making a driveshaft. Good luck, Jim

J D

Helpful tip - 12 BOLT CHEVY came with all the different yokes to install into all the GM A-body cars. BOP (mechanics3R) 1310, 1330 If you install the correct one you will not need a u-joint conversion swap.

Jim

ELY442

I did the same thing to my 70 Cutlass with a TH400. Mine came out of a 72 Chevelle. I changed the backing plates to a bigger brake shoes (11x2). I had a specialty shop made me a custom driveshaft with U-Joints which is a lot stronger than a factory. He even also install a chrome drive shaft loop for free

David DeCan

372 Moog. That's what I am talking about. Thank you.

Run to Rund

The upper ears for the bushings are closer to the top of the pumpkin on a 12 bolt than on the 8.2. You may have to grind the flange on one of the upper control arms where it would hit the housing.

monzaz

???? This is not correct... Sorry.
1964-1972 A-body all the same upper mount angles and points. 64 has smaller rubber diameter bushing and 65-72 are all the same.
Lower mounts are the same also the width of the housing is only OUTside the control arm brackets. 54.25 to 55.25