Checking for a Posi rear

I know if you turn one tire and the other tire goes in the opposite direction its an open diff. Also, if it turns in the same direction its a posi. On my 72 Nova SS, one tire turns at a time without turning the other. They both turn the drive shaft.


What the heck does that mean?:confused:

It's either an open, or a worn out posi. If it turns the drive shaft, it sounds like a worn out posi to me. That's what mine did when my Auburn gave out.

You have an open carrier. With a Posi or even a limited slip it should not allow you to turn one tire without the other going the same direction due to the design.

So if its a worn posi unit, using it as should not be an issue? Don't want to blow the money on a posi unit right now.

It's either an open, or a worn out posi. If it turns the drive shaft, it sounds like a worn out posi to me. That's what mine did when my Auburn gave out.

Sounds to me like you have an open carrier.

It COULD be a worn out posi, but it would have to be worn nearly to the point where it was collapsing on itself to act this way.

The worst, most worn posi unit I've seen still have about 15lbs of friction in the clutches to hold the two wheels together. Spinning one tire while up on stands still spun the other in the same direction. However when driving, you would swear it was an open diff. Best way to tell for sure is to pop the cover and inspect.

It shouldn't really hurt anything driving it around worn out...but most of them can be rebuilt and re-installed for ~$135 (DIY). Since you're re-using the same carrier and gears, you won't have to worry about shimming things to get the proper setup...just keep track of which shims came from which side.


If it is indeed a worn (stock) posi, they are incredibly easy to rebuild. When you buy the new clutch pack for the unit, make sure you order a set that comes with a set of clutch pack shims; this way you can shim up a certain level of pre-load to stiffen up your posi a bit better than stock.
My recommendation is to shim it as tight as you can, while still being able to re-assembly it. If you can't find the shims, Ratech makes them...part number 1801.

I know that. My feelings were that this was an open diff till I saw that turning one wheel in one direction did not turn the other in the opposite direction told me something else. What? That's what I'm trying to find out.

You have an open carrier. With a Posi or even a limited slip it should not allow you to turn one tire without the other going the same direction due to the design.

Thanks! That gives me some more to go on...

Sounds to me like you have an open carrier.

It COULD be a worn out posi, but it would have to be worn nearly to the point where it was collapsing on itself to act this way.

The worst, most worn posi unit I've seen still have about 15lbs of friction in the clutches to hold the two wheels together. Spinning one tire while up on stands still spun the other in the same direction. However when driving, you would swear it was an open diff. Best way to tell for sure is to pop the cover and inspect.

It shouldn't really hurt anything driving it around worn out...but most of them can be rebuilt and re-installed for ~$135 (DIY). Since you're re-using the same carrier and gears, you won't have to worry about shimming things to get the proper setup...just keep track of which shims came from which side.


If it is indeed a worn (stock) posi, they are incredibly easy to rebuild. When you buy the new clutch pack for the unit, make sure you order a set that comes with a set of clutch pack shims; this way you can shim up a certain level of pre-load to stiffen up your posi a bit better than stock.
My recommendation is to shim it as tight as you can, while still being able to re-assembly it. If you can't find the shims, Ratech makes them...part number 1801.

How can I tell if this has a 30 spline or a 28 spline?

You know the answer to this. Take it apart.

I would be willing to bet that a 10 bolt is a 28 spline.. The 30 spline I think would be the 12 bolt or 10 bolt from a larger car.

It always smells so good when that cover is removed from those old diffs. :eek: :eek:

Tommy:) Shoooooeeeee

It always smells so good when that cover is removed from those old diffs. :eek: :eek:

Tommy:) Shoooooeeeee

Well I don't think it's as foul as people make it out to be. I've done it a few times and it didn't bother me as much as, like...say a dead animal. Now those really stink.

Hey there is always the burn out test. Just light em up and see how much smoke you get.

My wheels turn in the same direction when I lift the rear off the ground.

However when doing burnouts my brother told me sometimes both tires spin and sometimes only one spins.

So I guess Im running limited slip.

My wheels turn in the same direction when I lift the rear off the ground.
However when doing burnouts my brother told me sometimes both tires spin and sometimes only one spins.
So I guess Im running limited slip.

The factory GM style limited slips didn't have as much friction holding the wheels together as say, a performance unit from auburn or eaton. This being said, it makes it very easy to burn up a stock GM posi unit.
A few burnouts where only one wheel spins can glaze the clutches or just straight wear them smooth. Once that happens it's going to act erratically or act just like an open diff. A stiffer unit with more force connecting the two wheels together will be less prone to letting one wheel slip, so this type of even is less likely to occur. However they can be a bit more dangerous to the average driver in wet/snow/dusty conditions. So you can see how GM's factory limited slip fits right in the middle...better traction overall, but not so much so that it makes driving more hazardous.

Of course, a long (30-40 year life) or city driving and lots-o-turning can spell disaster, too. Again, if your unit is worn out you can buy a complete clutch pack to give it better than new performance.

Well I don't think it's as foul as people make it out to be. I've done it a few times and it didn't bother me as much as, like...say a dead animal. Now those really stink.

I love the smell of 90wt myself...But I've been around shops since before I could walk. The only ones I hate tearing down are the stock ford limited slip rear ends. The additive they use to prevent chatter smells like rotten fish guts. However, it's probably the best additive I've ever used...I guess ford really got TWO things right...good differential additive, and they knew how to cirlce their problem!