I want to install a posi unit in my 10 bolt. I am fairly sure i have 2:56 gears and a 8.5 10 bolt. What is a good posi for my road racing needs....plus how do i tell which 10 bolt i have for sure and what spline the axles are.

If it's the original rear, it should be an 8.2 10 bolt.

http://www.novaresource.org/axle.htm


this is a good place to start.

The Eaton posi is the best all around limited slip differential for a GM car. Your car should have had a 8.2 10 bolt, but someone could have swapped in a Nova 8.5 10 bolt. You must find out which one you have before you start getting parts because they will not interchange. The axles in either one of these rear ends will have 28 splines.

you might as well pull the carrier out and look at the nubers on it , it will tell you right away what it is ; then count the splines on the axles.As far as road racing gears the 355 is pretty snappy graemlins/hurray.gif

I have found several posi units in mid 70's Trans Ams that I understand will bolt into the 72-75 Nova rear end. I have found a 69 and a 71 nova parts car but they may be the same 8.2 10 bolt that I have now. Does anyone know if either of these novas would have the bigger 8.5 10 bolt that I can use this posi unit with ?

http://www.novaresource.org/axle.htm

Novas started coming with 8.5" rears in '72.

I have snagged a 75 pontiac ventura with the 8.5 rear, not sure what gear ratio. I also have located three t/a's with the posi unit. How can you tell the gear ratio of the rear end while it is still in the car and not moveable ? I didnt see it on the posi tag.

If you can get to it, pop the cover and count the ring & pinion teeth, then divide the ring by the pinion.

The ring gear might have a set of numbers on it that will tell you right off which gear set they are.... again by telling you the tooth count of the ring and the pinion. You'll probably have to spin the ring gear to find the numbers though, which might be impossible.

Something like this: 1X GM 3790629 12.37 5 68
The 12.37 tells you that it's 3.08 gear set. 37 divided by 12.

Most TAs with automatics have 2.56 or 2.73 rear gears.

On the 8.5" rear end, the carrier break is between 2.56 & 2.73. Series 3 is 2.73-up, series 2 is 2.56-down.