hey guys i just bout an 86 z28 and was wanting 2 change the rearend 2 a posi rearend can u guys tell me wat size off differential it is (8.2, 8.5?) it was originaly a 5 speed but the people who ownd it before me replaced it with an automtic. wat kinda gears does it have factory?

 

If it has the stock 10 bolt it's a 7.5.

 

how do i kno if its the stock?

 

Pull the cover off the rear housing and check the ratio stamped on the OD of the ring gear and also check for the Limited Slip carrier. Anything else is going to be a pure guess from anyone considering your car's description.

 

The numbers are stamped on the ring and pinion:

41 : 15 = 2.73
41 : 14 = 2.93
40 : 13 = 3.08
42 : 13 = 3.23
41 : 12 = 3.42
41 : 11 = 3.73

JamesC

 

The 8.2 was discontinued in '72. The 8.5 was never installed in the '82 and later Camaro or Firebird. The only rear ends offered in '86 were the 7.5 10 bolt and the Borg Warner 9 bolt.

 

An 86 Z28 with a manual transmission normally came with 3.23 gears. If it has the G92 RPO code, it will have 3.73. Since the diff has been changed, the only way to know for sure what's in the diff is to pull the cover and count the teeth.

 

k thanks everybody!

 

Why would you want to count the gear teeth when the Mfg already stamps that info on the gear OD???...

 

Why open it at all?

Jack the car up, put it in Neutral & manually spin a tire.

Count the revolutions & the # the pinion yoke spins. You'll get about....
2 3/4 to 1 for 2.73
3 1/4 to 1 for 3.23
3 1/2 to 1 for a 3.42
3 3/4 to 1 for a 3.73
and so on.

Then theres a 3.08....the 9-bolt gears, etc,

 

That only works if you have a good working posi, where both wheels turn together in the same direction. If you have a standard differential it works different. The best way to do it with a standard differential is to leave one wheel on the ground and turn the other wheel 2 complete turns.