Found the code stamped into the rear of my 72 cutlass to be "SAB257 2"

From what I find its a 2.73:1 open rear. not sure about why it has a "B" or if the space then 2 means anything.

Is this just a regular gm 10 bolt?

Before I go through the $$ and effort to put in gears and a posi, how much power can it hold?

 

According to the '72 Chassis Service Manual, the SA indicates standard (not anti-spin) differential with 2.73 gear ratio. Ring gear size is 8.5", 10 bolt cover.

The "B" is the code for the manufacturer of the differential, but there is no chart giving the manufacturer names. There were four possible codes here, O, B, C, or K.

The numbers 257 indicate the build date, but the manual does not say how to decode them.

The example code shown in the manual does not have a digit off to the right like your 2 after the space, so there's no mention of what that means.

 

O- Oldsmobile
B- Buick
C- Chevrolet
K- McKinnon

These rear ends are known as the Corporate 8.5 rear end, which were started in 71 and are considered rugged. They were used in the A, B, C, F and X bodies well into the 90's in the big GM cars.

 

As for strength, they hold up behind big blocks putting out over 500 ft-lbs torque and going 12s in the quarter mile. If you need it beefier, you can get aftermarket 30 spline axle shafts and a spool.

 

Build it you will be fine... You need to determine what your car will be doing as to how to build the rear. For the street you do not need 30 spline axles or aftermarket axles for that matter.

JUST make sure whom ever builds the rear does it correctly...THIS is the most important factor to making the rear end last.

NEW parts always help in the strength area .... BUT getting them in correct is the biggest problem for failure.

Good luck.
If you need parts I can quote you out what I can do. I try to give forum discounts where i can.
Jim
J D