Hey guys, I need info on this one... I bought a complete read end from what was suppose to be a '96 Z28. I had a doubt from the beggining because it had cast iron calipers .(4th gen have aluminum ones, right?)

I believe it to be out of a 1984 Z28, but how could I know for sure? I found some codes on the right axle but no one (not even GM) know what those numbers are, OR what year it could came from. I'm 100% positive that it's a F-Body unit tho.

Also, on the axle at the end of the code thing, it's written 3.08, so I believe it ORIGINALLY came with this ratio, but when I removed the cover, and turned over the posi unit I saw 13:42 with a GM number 14065535 (cant remember about the number quite well).

So what's that 13:42?? as the unit been upgraded with a new one? I haven't dissembled it yet but I know for sure that there is 42 teeths on the ring, not sure about the pinion. Do 3.08s have 42 teeths ring?

 

Screw the codes. That's too much like work, and cannot be relied upon for accuracy. This is the sort of place that codes take you to; whaat it originally was, at best, but no hint at what it is now.

If you paid for a 96 model rear but got one with cast-iron calipers, you got it in the back door with no reach-around. Those are 88 or earlier. Pop an axle and count the splines. If it's got 26, you definitely have a 89 or earlier rear on your hands.

Gear ratios are easy. You've already done the work to get that; not too tough obviously. In the case you have, 42 / 13 = 3.230769230769.....

A 3rd gen rear is about 56¾" from the outside surface of one axle flange to the other. A 4th gen rear is about 60½".

 

I have paid only 200$ for it. and I now know for sure that it came from a 3rd gen (which I don't really care cause all I wanted was POSI and DISC setup).

It as 26 splines axles. now I just need to be sure of that gears I have. hmm.

 

 

What about 42/12? is this possible? it gives something like 3.5.. would this be 3.42?

what are the ring/pinion teeths ratio for a 3.42?

 

If it's 3.23 then I guess I can use 3.42 or even 3.73 on the same carrier... well that's a good news!

Thanks a lot RB!

 

sounds like a problem my cousin is having with his new/old rear.
he bought a rear out of a 87 GTA for his 91 cause he was told that it was a posi/disc brake rear, when he went to buy pads and rotors for it turned ot to be out of a 84 and no posi. now he is having problems with his speedo it reads faster than the real speed of the car. when we were unloading the old rear out of the truck we spun the yoke on the rear and both left and right sides spun in the same direction we tried to hold one side while spinning the yoke and couldn't hold it this would be a posi right? what can he do to correct the speedo? thanks in advance...

 

3.42 = 12 and 41

3.73 = 11 and 41

12 and 42 is not a good combo; the same teeth would end up touching each other all the time, and some teeth would never touch some other teeth. Gear ratios almost always involve at least one prime number, so that all pinion teeth touch all ring teeth equally.

GTA: take off the cover and look. All these "tests" for posi are inconclusive at best. Since this is a rear you just got, you need to be changing the fluid anyway; make a point of pooking at the gears and what's inside the carrier while you're doing the common-sense regular maintenance.

To make the speedo read right, get the correct speedo gears to go with the rear end ratio that you determine you have, while taking care of the above-mentioned overdue business.