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Posi not allowing slip in tight turns

3K views 20 replies 15 participants last post by  1966_L78 
#1 ·
Hi all,

I'm wondering if you could point me in the right direction on this?

My Chevelle has a GM 8.2 10 bolt rear, with some type of posi installed - not sure which one it has. I'd kinda like to keep it for the time being, until I'm ready to really upgrade.

Since I've had the car, it's had an issue where in tight turns (like in parking lots), the wheels will squeak. It's as if the posi is not releasing at all, and instead acts like a solid rear axle. Every once in a while, it will work like I'd expect (usually on hot summer days when the wheels probably grip a little better than usual - I can actually feel it break loose and slip, and the tires stay quiet), but almost every time, the tires will squeak on tight turns. It doesn't seem to have any other issues - no noise or anything.

From what I've read, it sounds like the issue could be that the friction modifier might either be missing, or not enough of it is present. So, I drained and refilled with a Red Line gear oil that includes it. In the one short drive I've taken, though, it doesn't seem to have changed anything - the tires still squeak in tight turns. I'm not sure if this means anything yet, because I only got the differential to where it barely feels warm.. but I was really kinda hoping that would just instantly fix it :|

My question is, does it take time for the new oil to work its way into the posi? Maybe I should just wait longer, and see if it starts working normally? If so, how long should it take?

If this oil doesn't do it, would it make sense to try adding more friction modifier? Is there something else that can go wrong with these, that causes them to be overly sticky like this?
 
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#2 ·
Did you look at the differential when you had the cover off? What did it look like? I'm thinking that you either have a standard differential that someone welded or you have a standard differential that has severe damage from doing one wheel burnouts. If it's damaged from one wheel burnouts then you may have one or both of the pinion gears welded to the cross shaft from spinning too fast without enough oil getting to them. One wheel burnouts are very damaging to a differential.
 
#3 ·
I did see the inside of it.. Although of course my brilliant self didn't think to try and spin it around and find part numbers in there 😞

I do not believe this was an open differential. Based on the pics I've seen, those seem to have big openings in the sides where you can see the gears. This thing has a closed shell that looked like.. every pic I've seen of a posi. Also, the old oil seemed to come out clean (albeit slightly darker than new - like a golden brown), and I didn't find any kind of metal fragments.
 
#5 ·
Hmm.. I should have been more clear. I saw inside the differential case, but didn't see inside the inner part at all (what I've been guessing is a posi) - the side facing me was competely enclosed. It looked like a solid metal cylinder in there.

I really should have spun it around to find out what I've got in there, but for whatever reason I just didn't think to...
 
#6 ·
As Freddie suggested you need another look inside. Drain your new oil into a clean container so you can re-use it. Rotate an axle until you can determine what you actually have and post up some pics. Sounds like you have a damaged open differential.
 
#8 ·
Since you just changed the fluid - before you drain it again, take it to a big parking lot, and then drive in a bunch of tight circles to the left, then a bunch to the right, then do a bunch of figure 8s. See if that helps work it in.
 
#12 ·
Freddie will comment, but I dont use Red Line, I use regular GL5 with GM posi additive. But if Freddie says RedLine is good, then its fine. I was only thinking when you pop that cover again and spin the diff around for inspection, you might consider using the "cheap stuff" with GM additive for the refill.
 
#13 ·
I had issues like this with a 57-64 Olds / Pontiac 9.3.. I was told to use regular 90 weight with two bottles of GM posi lube. I did and all the issues went away..

My 69 is all street so I prefer the regular rear end, they turn corners better. You really have to play with the pinion angle and use bags to get them to hook because once they spin all the torque goes towards the spinning wheel.. What a lot of people don't realize is both back wheels propel the car with the standard rear end not just one..
 
#14 ·
FWIW..I believe the old GM shop procedure when a GM posi rear was a"Grabby" was to drain the housing, refill with kerosene, do 5 or 10 right circles and 5 or 10 left circles . STOP, drain out ALL of the kerosene and refill the rear with proper posi-lube. Do not drive anywhere without draining the kerosene unless you desire to replace your bearings. The concept was the kerosene, being of very low viscosity, allowed the "poorly" lubricated posi-clutches to re-bed their surfaces, and helping eliminate / dissolve any surface build up on the posi-clutches. My poor memory cannot recall if it was 5 or 10 circles in each direction. Might find it in an older shop manual or service bulletin.
 
#15 ·
So, I finally got the cover back off, and it looks like someone replaced the spider gears with a mini spool (I wasn't even aware there was such a thing). I'm not sure why someone would want one of these on an 8.2 with 3.08 gears, but there it is :laugh:. Either way, I guess I can stop wondering why the wheels seem to be stuck together!

Any suggestions on what to do next? This car is meant for the street, so I'm not planning on subjecting it to hard launches with sticky tires.. but I do like stepping on the gas and getting that "kicked in the back" feeling! Is the 8.2 worth upgrading in these circumstances (an actual posi that fits it looks to be about $500), or should I just go all the way and get a new axle?

If I were to replace the whole thing, do you guys have any thoughts on where to look? Are there used ones out there, or ones currently in the junkyards that people are modifying to fit? The new built-to-fit ones (from places like Moser) are quite pricey - but maybe that's just what it costs to get a solid rear end?
 
#21 ·
Any suggestions on what to do next? This car is meant for the street, so I'm not planning on subjecting it to hard launches with sticky tires.. but I do like stepping on the gas and getting that "kicked in the back" feeling! Is the 8.2 worth upgrading in these circumstances (an actual posi that fits it looks to be about $500), or should I just go all the way and get a new axle?

If I were to replace the whole thing, do you guys have any thoughts on where to look? Are there used ones out there, or ones currently in the junkyards that people are modifying to fit? The new built-to-fit ones (from places like Moser) are quite pricey - but maybe that's just what it costs to get a solid rear end?
For a 64-72 Chevelle, you have to stay with another 64-72 A-body, as those will be the only ones with the correct brackets. Either that or aftermarket like Moser or Currie...


IMO, IF you are going to rebuild/upgrade, but not all the way to aftermarket, then I'd either look for 12-bolt or an 8.5 10-bolt. Most costs (bearings, posi unit, labor) are going to be similar between the 8.2, 8.5 and 12-bolt... Its mostly just the cost of buying the housing.

Make sure you know what you are looking at too... 1973 and later A-body and G body look similar with the upper ears, but they do not swap over... I see a lot of people advertising "track" 12-bolts as "car" rearends...
 
#16 ·
Because of the suspension design, specifically those two upper rear suspension arms and the attachment ears cast into the centersection, ....there is no simple, cheap wrecking yard alternative. About as close as it gets would be a 8.5" 10-bolt out of a '71-72 Buick Skylark, Olds Cutlass...but those a pretty hard to find, and still likely to need both an posi unit and a better gear ratio so it really isn't going to save you much if anything at all.

Otherwise, you're looking at trying to find a 12-bolt someone is selling, or step up to a aftermarket 12-bolt or Dana 60 with the correct cast ears, or a 9" Ford made with the proper ears welded to the steel housing.

Or, you put a posi and gears in the 8.2" if you don't plan a bunch of dragstrip abuse.
 
#18 ·
I checked that link out and in more than a couple of those ebay ads they mention an 8.6 rear .So what the heck is an 8.6 GM rear ?
 
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