The Stovebolt Site Forums The Shop Area Engine Shop positrac rear-How to tell?

I have a 57 GMC 3/4 ton and I'm hoping to do a ring and pinion swap. Called Patricks and they were very helpful. I told them I had a Spicer 60 rear with 4.56. I was under the impression that it was positrac as I thought I read somewhere that if you jack up the rear and turn one wheel the other will turn in the opposite direction. Do I remember wrong? The guy at Patricks said that posi will both turn the same direction. Also, to confirm the existing ratio I counted revs of axle with both wheels off the ground. Right at 2.25 per 1 wheel revolution. 2.25 x 2 wheels = 4.5 (close) Does that sound right or am I way off on calculation procedure? He said to try with only one side up so I did. 2.25 and wheel would only turn in reverse. Does this sound correct?
Any help to clarify my fuzzy thinking would be greatly appreciated.
Also, He said the carrier would have to be changed to accomodate the gear swap (3.54). Would a less drastic gear change make a difference in the carrier change? Sorry for the long post. Thanks again.

Sounds like you have a 4:56 and it is a limited-slip. one wheel turns so far and then they both lock for traction to both.
Pproblem with the Dana/Spicer rear ends is that if they start to move the side bearings, the differential side bearings can start to wear the sides of the case, making it a piece of scrap meatal as it would be cheaper to replace the whole rear end than to have it machined :rolleyes:

The limited slip rear ends I'm familiar with (GM, Auburn, PowerTrax) all turn the opposite wheel in the same direction. The open rear ends I've dealt with spin in opposing directions.

Im here in Moore County Texas. I would highly doubt you have anything but an open diff. Those things werent common till the late 60's on.You are in the same boat i am, at a certain ratio you have to make the case change to gear it up.

Thanks to all for the input.

The rear tires will spin in opposite directions on all open diffs. However, the same will occur with some limited slips as they only lock up when sufficient velocity difference exists between the two axles.

Short of pulling the rear cover, the best method for determining if you have limited slip or locking rear axle is the functional test: Find a paved road with a loose gravel shoulder. Pull off far enough to have one tire on pavement, the other in the loose gravel. From a stop, hit the pedal fairly hard. With a properly operating limited slip, you will have near-normal acceleration. A shower of gravel and very little progress indicates an open rear end (or, of course, a trashed posi!)