[kyle_blake]

Hi

I have a corporate gm differential with 390 posi gears. I've jacked it up and removed the wheels and drums. Both my drums had some warpage but the pads still had 6/32 left on them so about 60% left. I decided to get some new drums.

So question for you, I put the new drums on and pads aren't touching them. Which is fine, I know how to run the star wheel till slight drag but what has me concerned is:

When I turn the drum on the axle it's easy going for 80% of the revolution then gets very difficult to turn. Then you move through that 20% more and it gets easier. Same thing if you keep turning it. Isn't it suppose to just be free all the way around?

Is that normal? This rear end is a c-lip rear end. E-brake is off, vehicle in park. The u-joint is not moving as i spin the drum manually.

Hopefully this is normal and it's just me?

[STEELCITYFIREBIRD]

Go in the car and press the brakes once to center up the shoes. Bolt the brake drums up tight with wheels on, or lugs first. See if that makes a diff for ya. HTH

[Tom Vaught]

If you jack up both sides of the axle and turn the wheel "Forward travel" on each side, maybe you can pick out which side is causing the 20% binding condition. With a Chebby style clutch type rear you might be able to identify the issue more easily.

Tom Vaught

[kyle_blake]

The parts store only gave me one drum at the moment. So I'm only turning the passenger side drum assembly. I will relocate the drum to the other side adjust pads out a bit and try turning it, see if it suffers the same difficulty turning.

The shoes aren't part of the equation as to why the differential is difficult to turn the 20% of the revolution, so hitting the brakes I don't think will make a difference.

I'll go try the drum on other side and report back Tom.

Thanks everyone.

[kyle_blake]

Same thing other side, both directions difficult to turn 20% of the revolution. I guess next step pull dif cover and inspect. Hmmmmmmm

[arcitech]

Originally Posted by kyle_blake

Hi

I have a corporate gm differential with 390 posi gears. I've jacked it up and removed the wheels and drums. Both my drums had some warpage but the pads still had 6/32 left on them so about 60% left. I decided to get some new drums.

So question for you, I put the new drums on and pads aren't touching them. Which is fine, I know how to run the star wheel till slight drag but what has me concerned is:

When I turn the drum on the axle it's easy going for 80% of the revolution then gets very difficult to turn. Then you move through that 20% more and it gets easier. Same thing if you keep turning it. Isn't it suppose to just be free all the way around?

Is that normal? This rear end is a c-lip rear end. E-brake is off, vehicle in park. The u-joint is not moving as i spin the drum manually.

Hopefully this is normal and it's just me?

Sounds like you are feeling a worn out posi unit. Put the trans in neutral and try again.

[TedRamAirII]

Maybe put 2 lug nuts on to be sure the drum is square to the axle flange. I hope the drums are "round"

[kyle_blake]

I put it into neutral. Spun it forward, difficult to move, requires more strength but the tension all the way around is the same BUT after several revolutions or so... It spins very freely almost as if it's slipping, like where did the tension go? (I notice u-joint not moving when this happens ) - then it catches after a couple more revolutions again and it's very hard to move. This is on the drivers side.

I tried the same procedure in reverse direction and it was identical.

the one drum for sure is perfectly round, spun up on machine, brand new.

[STEELCITYFIREBIRD]

"The shoes aren't part of the equation as to why the differential is difficult to turn the 20% of the revolution, so hitting the brakes I don't think will make a difference."

If the problem seems to be inside the differential, why put the drum on at all?

[arcitech]

From your first post: [COLOR="Red"]"I have a corporate gm differential with 390 posi gears." [/COLOR=Red"]When the posi is connecting, you are turning both axles, if for some reason (worn out) it loses tension, then your posi is going bad. Adjust your brakes when you get the rest of your parts, put the tires back on, and do a burnout! If only one tire smokes, you have identified the culprit!

[arcitech]

Originally Posted by arcitech

From your first post: "I have a corporate gm differential with 390 posi gears." When the posi is connecting, you are turning both axles, if for some reason (worn out) it loses tension, then your posi is going bad. Adjust your brakes when you get the rest of your parts, put the tires back on, and do a burnout! If only one tire smokes, you have identified the culprit!

Do you know what type of posi-traction unit you have? Cone/clutch/Eaton...?

[kyle_blake]

STEELCITYFIREBIRD -> The task at hand was to inspect brakes, and part of adjusting brakes (I'm learning brakes in school / we haven't taken differentials and I know zero about them aside of 2 tire burnouts and one tire burnouts ) is to rotate the drum and feel for shoe drag. While doing that is when I noticed weird differential drag.

I know that my differential isn't a pontiac differential but I haven't gotten 100% confirmation as to which one it is. I posted on her some pics and the consensus was I believe that it was a corporate bop rear end, could be oldsmobile, I bought the car with it in there.

A friend told me several years ago after I did a burn out that I left only one tire mark, I always assumed it was because of old tires or I was on an angle. It could very well be worn out.

I'll call him , he says it was just one tire moving ... but I know for sure mechanics have said it is a posi unit.

So must be worn.

[steve25]

Unfortunately the odds are and just like Murphy's law , your Posi is toast!

[Tom Vaught]

Based on the latest info you have provided, I would agree that the Posi unit needs to come out of the vehicle.

A Factory Cone style Limited Slim Differential by its design TRIES to turn both wheels evenly every time. A Clutch type unit that shows tire rubber on one side of the car is wounded if not failing. A Friend with a 65 GTO had a Eaton friction plate Posi-unit that failed after one weekend at the track with good tires.

You might want to read this article as it gives a couple of good tips and some diagnostic advice. http://www.hotrod.com/articles/10-an...on-improvment/

Tom V.

I am assuming the 20% tight deal is a spot where the discs bind together and do restrict motion somewhat keep the wheels from moving. Not working as a Posi, just binding.

[kyle_blake]

Thanks everyone for your help. Now may be the time to drop down to 331 or 342 or 355 gears if at all possible and get posi working again. check all axle bearings and some new fluid and away we go! Now just to find out what differential shop can assist me. Maybe I'll take it on under their guidance if I'm lucky enough.

['ol Pinion head]

put up a couple good pics of the center of the rearend & I can ID it, have racks & racks & a big pile of original GM muscle era rearend housings & all of them are getting moved to their new home this next week.

Of the original late '60's muscle era A-body rearends, only the PONTIAC 8.2 10 bolt & the '68-70 BUICK 8.2 10 bolt originally used a cone type posi carrier.

If the rear is a corporate 8.5 A-body rear, I can ID all three versions. There is a possibility, if that is the case, & it's a '71, that the posi carrier is a Warner Motive 2 pinion cone type. Have one of those as well...

[oldblueponcho]

Originally Posted by kyle_blake

Thanks everyone for your help. Now may be the time to drop down to 331 or 342 or 355 gears if at all possible and get posi working again. check all axle bearings and some new fluid and away we go! Now just to find out what differential shop can assist me. Maybe I'll take it on under their guidance if I'm lucky enough.

You're in Vancouver, so I'm gonna say that I.W.E. Rear Ends Only is about the best around. good prices too.

[kyle_blake]

A fellow named Keith helped me a long time ago, he wrote:

>>Yes, your rearend is an Olds rearend. Well, it's actually a
>>corporate
>>rearend, in that it was used on Oldsmobiles, Pontiacs and Buicks
>>of the
>>same era. It's the 8.2 10-bolt. I've got the same one in my '67
>>that
>>came from a '67 442 with a 4-speed.
>>
>>Keith
>>
>>
>
>
>

With this rearend, there is no 'Pontiac' or 'Olds'...it's GM
corporate....that means the exact same rearend was used on various GM
makes and models, unlike other rearends that were specific to a certain
make. Take a look at the following link for more info:

http://www.oldsmobility.com/old/reardiffs.htm
http://wildaboutcarsonline.com/cgi-b...=9990382924295

http://www.442.com/oldsfaq/ofdif.htm...ential%20Codes
http://forums.maxperformanceinc.com/...d.php?t=795793

Hope this helps.

Keith

MORE INFO ON MY DIF:
Rear drivers side of DIF
12345
67890

Rear passenger SIDE
Horizontal 2

FRONT DRIVERS 68D
Front PASSENGER SIDE 1385724-F

[kyle_blake]

Hi, just lost my message, stupid website.

So in 1st gear, slight brake stand, two tires spin, let off brake and go, one tire burnout. In reverse, two tire burn out when i quickly stab the throttle for a quick second to back up

is there upgrades for the posi unit? can i just replace it ? I was thinking of buying the kit and with the help of a red seal mechanic friend, try and fix it?

Any videos? Rebuild kits you know of? Or potential R&R as an option?

[Cardo]

I've had 2 of these in 2 different cars and no problems out of them at all. If you're just running a car on the street, this fits the bill perfectly. http://www.ronsmachiningservice.net/...line-19674-10/