My friend tells me my 72 cutlass convertible has a posi in it . I'm about to pull the aluminum cover drain and fill how do I know for certain it's a posi and if so what is the additive I use in it? Thanks ps I don't see the w27 stamp on the cover

 

If you jack up the rear end spin the wheels if both move togeather same direction it's a posi if one turns forward and the other reverse then its not if it is a posi there is a gm posi additive

 

The W27 was only for the aluminum 3rd member that was available on the W cars, the cover itself was also an option, but posi was not necessary to get it.

 

so is it safe to assume there are only two types of rear end posi and limited slip?

 

There is only one kind. "Posi-Traction " and "Limited-Slip" are two different trade names used by various car makers to refer to controlled traction differentials.

 

No, there are two types

1) "open" where one wheel spins and the other does not.

2) Various names for controlling slip- positraction, Limited Slip, etc. Various mechanisms are used also, but ALL involve more than just some spider gears that are totally free to move.



You might want to watch the testimony of Ms. Vito in "My Cousin Vinny" also

 

With all due respect, there are 4 types, sort of.

1. Open.
2. Limited slip
3. Locking.
4. Direct gear transfer.

A locker can be locked by the car or by you, either electrically, cable actuated, or turning some bolt.

The reason I included direct gears is that a lot of the old 4 wheelers just had a straight gear set with no differential gears. I guess the closest thing cars have is a spool for draggers.

 

The cover for the rear must be after market it has no numbers or markings on it that I can see . I guess I'll know for sure when I pull the cover and see what type of gearing is in there if I remember the open rear end will have the main gear and the two spider gears?

 

Smoke the tires in a straight line & see if you leave two stripes.

 

Lol now that's a straight up answer and you can bet I will when I get it running good

 

The rear is stamped on the passenger side axle tube, that is where you will find out what gears came with the car originally, and whether it is a posi.

 

Positraction,Limited-Slip,Sure-Grip,Safe-T-Trac,etc. All the same thing.

Non-posi,Open,Peg-Leg,One-Wheel-Wonder,etc. All the same thing.

Now there are different types of posi units: clutch style,cone style,coupler,helical gear.

 

The obvious correct way to ascertain the information!

"With all due respect, there are 4 types, sort of.

1. Open.
2. Limited slip
3. Locking.
4. Direct gear transfer. "
=========================
OK, sure, but for most purposes, by far the most common is 1) Open, followed by a few 2) "posi" or similar name not-open differentials... the other 2 are few and far between. The chance of finding anything other than 1 or 2 in a std issue car is right about zero.

When you pull the cover the guts should show right away if it's open. Your tooth count will be stamped into the OD of the ring gear, like
"11 39"
Divide the two numbers to produce the gear ratio- in this case 3.55

 

You are correct on what is never found. I was just talking some theory and shop.