I have a 72 10 bolt open rear end with 2.73 and would like to put new gears and a posi in it. Is it a 8.2 or 8.5? What are you measuring to determine this? The car will have a M21 and a mild 402 any suggestions on a gear ratio? I was thinking on 3.42 or 3.55. One last question, should I look for an aftermarket or GM posi? Thanks for any help. Mike

The 8.2 or 8.5 figure is actually the diameter of the ring gear. 72 being the change over year you could have either one. I think the bolts holding on the ring gear are a different size so that is one way to tell. I don't know what the sizes are though but maybe someone else here will know. As far as gear ratios go, If you are using a M-21 you will want a deeper rear gear. 3.55 or maybe even 3.73. Depending on how hard you are going to push this combo (and how much money you want to spend) a GM posi will be fine but a used one may need rebuilding. A very good aftermarket unit is the Eaton posi but will cost a little more.

Here's a site to help you determine which rear you have....look at the rear of the differential, then look at this page. It will tell you if it's an 8.2 or an 8.5 rear.

http://novaresource.org/axle.htm

The 8.2 has a 1 1/8 pinion nut and 3/8 ring gear bolts with 9/16 heads. The 8.5 has a 1 1/4 pinion nut and 7/16 ring gear bolts with 3/4 heads and left hand threads. The new Eaton posi is better than the original Eaton used in the 8.2, or the Traction Lock used in the 8.5 10 bolt. The 8.2 will use a 3.55 gear and the 8.5 will use a 3.42 gear. You will like either one of these on the street. You might want to think about some after market axles and other parts to make it a little stronger with the big block and 4 speed.