Classic/Antique Car Repair: GM Rear and posi ID, donor car, rear wheels

Question
Dick,

I am going to look at a rear end upgrade for my project car. The seller claims the the rear is a GM 10 bolt that has 4:11 gears and a posi. Unfortunately the donor car does not drive. I know how to prove the gear ratio, but how can I tell if it has a posi without actually doing a burnout? I dont want to pull and pay for a rear that is not what I want. Please advise.

Thanks
Scott

Answer
First, I have to apologize and explain why it took me so long to answer - I JUST got this!!.  The system has some problem and is not sending questions to me when they receive them.

The way to tell if it has a posi is to jack it up so that both rear wheels are off the ground.  Release the brakes so that the wheels turn easily.  Put the transmission in park, or if the donor car has standard transmission, put it in low or reverse, assuming the clutch is good and the engine is in it.  (The point is, you have to prevent the driveshaft from turning.)  Then turn one rear wheel and see if you feel a LOT of drag, and notice if the other side wheel goes the opposite direction. If you don't feel much drag, and the other wheel goes the other direction, it is an open rear end.  If you do feel a lot of drag, and/or the other wheel goes in the same direction, you have a posi.

There is also some marking on the case, but I do not know what that marking has to be to indicate a posi, and even if it has a posi, you won't know if it is in good shape without doing this test.

Dick