I have a 1970 cutlass supreme, I recently decided to change the rear diff fluid, I bought a gasket for a 1970 cutlass supreme rear end, i found it is incorrect, and from what i read this is a 1971 rear end. I cannot find any numbers on axle tubes, casing has a number cast in it 407296, and by looking at a diff calculator the gears are 2.73. Please tell me if my gear guess is correct based on numbers in photo of ring gear. Also is this a posi unit? From what I read it is, but the only photos I find of a posi unit has the s clip in the center of gears. I dont know much about rear ends, just how to change fluids, I would like to learn some. Any advice or guidance is greatly appreciated. I'm pretty sure I have seen 2 tire marks when I have spun the tires in driveway, but I haven't looked real close at them, as I don't beat on the car too much.

 

That is a 1971-1972 8.5" 10 bolt rear with an open differential and 2.73:1 gears. The block "O" on the forward part of the center section between the parallel ribs is the key identifier for the year / make. A limited slip diff would have the S spring and clutches on either side as you saw in the pictures you mentioned.

 

Fun71, thank you very much for the info, its nice to know what I really have.

Scott,