I want to buy a posi differential for my 71 cutlass, it has the 8.5 10 bolt. there seem to be two options in terms of splines, 28 and 30. which do i buy? everything in the rear end is stock spec except for the 3.73 gears i had installed in the summer. any ideas? thanks, all.

 

Corporate 10-bolts are usually 28 spline on older cars. To be sure, remove one of your axles and count the number of splines on the carrier end of the axle.

For posi's you have a lot of choices, both factory and aftermarket. I have used the richmond "power trax" in the past that fits into your existing open carrier. This is good for mild to robust use. Just be careful on slippery roads.

Others will require you to replace the gear carrier with a posi-traction carrier that matches your ring gear, but are more streetable and durable for heavy use.