Open or Posi?

[02_GMC_97_FIII]

Finished parting out a truck that the owner was sending to the scrapper. Got a bunch of parts including the full rear axle off of a '71.

How do I tell if it is an open diff or is a posi unit? When both tires were raised off of the ground and turning one wheel by hand, the other stayed in place. (no reverse turn) Haven't had a chance to pull the cover yet.

The guy insisted it was posi however wasn't positive, ended up getting the axle thrown in for free so couldn't argue with that. Just trying to figure out if I will soon no longer have a peg leg under my truck.

Thanks!

[Walts_restoration]

simple, bolt it to your truck, go find a new stretch of pavement, and lay down the hammer!!!! either it is or isnt. Also by popping the cover you can find out, the posi unit is more complex, someone can explain it better.

[JAKES 68GMC]

You can also jack the rear end up, and turn one wheel. If the other wheel turns the same way, its a Posi.

[krue]

Pulling the cover is the surest way to tell.

[Ackattack]

what exactly is meant by "posi" I think its an old school term, cause now adays I'm familiar with limited slip diffential, locker, and spool. I'm kinda thinking a posi is the same as a automatic locker, am I right?

[71chevylowrider]

If it's a standard GM 12 bolt rear end then if it is posi it will likely be an Eaton limited slip, which in essence is what we call posi, even though it's not a locker or a true locked type posi unit, it does do dual rear tire burnouts. Now, here's what they will look like. the pic is an Eaton Limited slip for a 12 bolt truck.

[jfalesi]

I'm no expert on rear ends, but I believe "Posi" is short for Positraction, which is what GM called their limited-slip-diff back then. It also provided the last bit of evidence for the court case in that old movie with Joe Pesci and Marisa Tomei (speaking of rear ends...)

[Select]

My 68 has 4.11 posi in it...as I was told when I got it. When I take right hand turns and gas it instead of feather it I get wheel hop. This doesn't happen turning left. This makes me think the spiders aren't welded. AKA poor mans posi. What could cause this? Sorry to hijack....Mikey

[Ackattack]

wheel hop is more of a suspension thing, not differential thing.

[albertarose]

when turnig tight radius turns at slow speed the clutches grab intermittantly especially if you are towing its not really a wheel hop but more of a jerking sensation you can hear it too ..

Originally Posted by Ackattack

wheel hop is more of a suspension thing, not differential thing.

[eedgell]

I think the "posi" is short for positraction, which means positive traction to both wheels. If you lift one wheel off the ground, and the posi unit is in good shape, you should not be able to turn the wheel that is off the ground. The posi works on centrifugal force. At about twenty miles per hour the clutch system disengages, and then operates like a one legger does. The best way to tell is to pull the cover, but the test I listed above works well if the unit is working properly.

[02_GMC_97_FIII]

Pulled the cover tonight and it's a peg leg. No complaints though it was free and after pulling the drums noticed they had recently been turned and had brand new brake hardware, springs, pads, etc. Has 3:73 gears in it, not bad.

Think about maybe rebuilding this one with a posi unit, swapping drum for camaro disc setup, and swapping it in. For now it will rest in the corner of the garage, on to tinkering with the rest of the stuff I picked up.

Thanks!

[albertarose]

I dunno about that ..Mie left 2 patches of rubber for a hundred feet and when it shifted to second at 5500 rpm I was deffinatly going faster than 20 mph LOL :rndll:

Originally Posted by eedgell

I think the "posi" is short for positraction, which means positive traction to both wheels. If you lift one wheel off the ground, and the posi unit is in good shape, you should not be able to turn the wheel that is off the ground. The posi works on centrifugal force. At about twenty miles per hour the clutch system disengages, and then operates like a one legger does. The best way to tell is to pull the cover, but the test I listed above works well if the unit is working properly.

[Morgsie]

Originally Posted by albertarose

I dunno about that ..Mie left 2 patches of rubber for a hundred feet and when it shifted to second at 5500 rpm I was deffinatly going faster than 20 mph LOL :rndll:

No, eedgell is right. Try punching it at highway speeds, it will not engage the other wheel. My Impala has an "original" posi, if you want to call it that. Regardless of speed power is transfered to both wheels. That means passing people on icy/wet roads at highway speeds can be pretty exciting if you don't know what to expect. This is why they developed the "locker"; it will not engage at fast speeds which means you wont be doing dough nuts into the ditch or other people.

[LONGHAIR]

The posi works on centrifugal force. At about twenty miles per hour the clutch system disengages, and then operates like a one legger does. The best way to tell is to pull the cover, but the test I listed above works well if the unit is working properly.

This is why they developed the "locker"; it will not engage at fast speeds which means you wont be doing dough nuts into the ditch or other people.

The diff that you guys are talking about is GM's Gov-Lock. They were not available until '73...as replacement for the original "posi-traction". "Posi" is a shortened/slang and often mis-understood/mis-used term. The Gov-Lock was supposed to be safer and more forgiving, but they are not as strong. The little clock-works guts will come apart if you have more power and/or bigger tires.
A "locker" is a totally different animal.
This is the best write-up I have seen...

Gov-lock(G/L) Used by General Motors in 73 & newer trucks that came with an option locking / limited slip differential. It was used in 7.5", 7.625", 8.5", 8.6", 8.875" 12 bolt truck, 9.5", 10.5", and now in the new 11.5" differentials. The smaller designs are fairly weak and the 8.5" and 12 bolt truck designs are especially weak. The 9.5" and 10.5" 14T units are fairly strong and will hold up well to towing and moderate abuse. The Gov-Lock is designed to act as an open differential during normal driving with no force being applied to clutches. This fact that no force is applied to the clutches keeps the clutches from chattering during normal every day driving. When either wheel slips and spins approximately 100 rpm faster than the other a governor assembly senses the difference in speed and activates a ramp system. The ramp system causes the clutches to engage and transfer power to the wheel with traction. One other feature of the Gov-Lock is that it will not engage at vehicle speed above 30 mph. This no lock-up feature keeps the vehicle more stable at higher speeds where both wheels spinning could cause an inexperienced driver to spin out or slide off of the road. This is a great design in theory and works as designed in a stock vehicle that is not abused. The problem is that most of us modify our trucks and many of us abuse them. When this unit is asked to deliver under extreme conditions it tends to self-destruct. The force generated by the ramp system that is used to lock-up the clutches works well, except that same force has to be contained by the case and the case is not always up to the job. (factory)

[msgross]

posi will spin both wheels (mud or burnout) and then lets one wheel turn faster in a corner with it's clutch pack inside. That keeps from having wheel hop like in a locker....

[71chevylowrider]

For the record, when your shopping for a posi unit (limited slip) in a GM 12 bolt truck rear end, try to stay clear of the Gov lock units, they have too many little weak parts that will not hold up to the kind of abuse us gear heads like to give em, for a stick it in & drive it, try to get the Eaton unit, if you need something even beefier than that look at the Moroso brute & the Auburn units.

[02_GMC_97_FIII]

Thanks for the info '71, I do like to put my foot into it so will shy away from the Gov lock units. No reason to buy, break, and then have to upgrade.