70' Cutlass Supreme Holiday Coupe - Differential ?'s

Old March 1st, 2019, 06:37 PM
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70' Cutlass Supreme Holiday Coupe - Differential ?'s

Trying to sort out what type of differential I have and learn some things as well. I'm a novice to this topic so I'm trying to learn and understand as much as I can. From what I believe to be correct, my 70' CS stock rear-end would be a 10-bolt BOP style (12-bolt cover) type O non-posi rear-end with 2.73 gears but this is purely speculation from trying to piece together all sorts of info found on the web. I've yet to find the codes or stampings on the axle tubes but when it warms up i'll go out and wire brush the rust surface and see what I can find. The cover seems to match what others have described as the BOP style with the 2 'ears' on the bottom of the cover. Am I right on that so far?

It's a family cruiser and not a street/strip car but there have been some engine modifications (engine pulled and completely rebuilt, decked/honed .030 over, forged pistons, cam, #6 heads rebuilt with adjustable rockers (CompCams), aluminum intake, Holley 750 DP, MSD, etc). Is there a budget friendly upgrade that I could do to the rear-end and if so, what are the benefits of doing this? Or, should I just stay stock with it and call it a day. I certainly don't have any complaints with the car, but would upgrading the rear-end give it a little more get up and go?

If an upgrade is done, what is involved in that?
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Old March 1st, 2019, 07:13 PM
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You likely have 2.56:1 rear gears. My first car was a 1970 Supreme 350-4bbl, dual exhaust, Hurst Dual Gate shifter, and the base (no cost option) 2.56 rear. I would have loved if it had a 2.78 (2.78 was an offering for the O-Type; the 2.73 ratio was used with the 10 bolt rears).

The budget friendly upgrade for a 2.56 open O-Type is swapping in a different rear with better gears. Been there, done that.

And yes, changing the rear gears will make a world of difference.
Back in the 80s, I changed the 2.56 rear for a 3.08 rear and it felt like I had a new engine with more horsepower.
Later I changed that 3.08 rear for a 3.55 rear and again it felt like I had a new engine with more horsepower.
2.56s are great highway cruising gears but suck for performance.

FYI, the O-Type was made for a short period of time (67-70) and as such parts are not as easy to locate as parts for an 8.5" 10 bolt rear. And the O-Type came from the factory with a limited number of gear ratios: 2.56, 2.78, 3.08, 3.23, 3.42, and 3.91. In addition to that, there were different carriers for the different ratios: 2.56 and 2.78 used one, 3.08 and 3.23 used another, and 3.42, 3.91, and dealer available 4.33, 4.66, and 5.00(?) gears used another.
So to do a gear swap in an O-Type, you would have to get a set of gears (either old GM or aftermarket) and a carrier (either old GM or aftermarket). The aftermarket gears are made to fit the 3.08-3.23 series carrier. The only aftermarket carriers available are made from modified Ford 8.8 carriers and are made to fit the 3.08-3.23 series gears. .

Swapping rears is easy - much easier than swapping gears: get a new rear, either an O-Type, a 71-72 8.5" 10 bolt, or a Chevy 12 bolt, and an appropriate driveshaft.
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Old March 2nd, 2019, 05:02 AM
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The only rear axle installed in a 1970 Cutlass from the factory was a Type O, period. That is not a "BOP" axle, it is unique to Oldsmobile A-body cars. It may have 12 bolts on the cover but it only has ten bolts holding the 8.5" ring gear to the carrier. Nothing interchanges with parts from other GM axles. It's certainly possible that the axle got swapped over the last half century, so be sure to positively identify the axle before buying any parts for it, but the Type O was the only one installed at the factory.
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