Rear End ID?
 

Its a 1969 GMC 12 bolt.
Is it a Posi?
Why is that roll pin so long in photo 2?

Re: Rear End ID?
 

GM Gov-loc posi. Pin is an anchor roll pin to catch/stop a fly weight. Not a preferred posi although they were not all that bad in a 12bolt. There are better posi centers out there. Tru-trac being at the top of the lsit

Re: Rear End ID?
 

Thanks Kwmech,
I did the trick of jacking up the rear, putting it in neutral then spinning one tire. The other wheel spun the opposite direction. I was told that meant it was not posi, what do you make of that?

Re: Rear End ID?
 

That is the personality of the gov-loc. It will not lock until 1 tire spins about 100 rpm faster than the other, then the spider gear spins the flyweights locking everything up

Re: Rear End ID?
 

Thanks, That is very good info.
My 72 was the same and I always wondered about the wheel spinning direction test. Now I know.
Were most of these old trucks with posi from the factory a Gov-Loc style?

Re: Rear End ID?
 

The Govlock came out in the late 70s I think, anything before that was a traditional posi..

Re: Rear End ID?
 

Mine is a 1969 GMC

Re: Rear End ID?
 

Gov locks came about in 73...axle could possibly have been changed along the way..
Copied this...
. In the 1973 model year, General Motors decided to start using the Gov-Lock in trucks, in an effort to eliminate chatter associated with conventional spring preload clutch type positractions (limited slip differentials). This was the one of the few solutions available using 1970’s technology. Thus, for GM trucks produced after 1972, the only traction-enhancing option was the Gov-Lock (produced by Eaton). These governor actuated units provide a locking differential without chatter. Great for the average truck owner, but not for those who really use their trucks off-road. Often referred to as a “time bomb” or “Timex Posi”. These locking differentials work fairly well for occasional use when traction is of small concern, but not when power transfer and traction are the main goal.

Re: Rear End ID?
 

The trucks original SPID says the "Rear Positraction" code is "G80".
Eaton calls the gov-lock "Eaton G80".
Is this just a coincidence or was this 1969 GMC originally equipped with the Eaton G80?

Just curious about the history.

Re: Rear End ID?
 

I think GM recycled rpo codes thru the yrs...

Re: Rear End ID?
 

Ok but, supposedly the Gov-Lock started to be installed in our trucks in 1973. This SPID is 1969. Of corse the code could just be a coincedense.

Re: Rear End ID?
 

My 72 Blazer originally had a gov-bomb, until pieces of it came through the rear cover.

Re: Rear End ID?
 

I guess more important than the history of my rear diff is what should I do about it?

It's got a pretty stout 393 stroker. It's gonna be a cruiser and never haul anything. But I'm sure I'll have to do at least a few burn outs.

Should I replace it, and what with?

Thanks

Re: Rear End ID?
 

Tru trac... you wont regret it

Re: Rear End ID?
 

Eaton Detroit Truetrac or Currie Enterprises Detroit Truetrac?
The Currie from Summit is about $300 more

Re: Rear End ID?
 

Mine is Eaton...never dealt with the Currie..

Re: Rear End ID?
 

Mongo, thanks for helping me out on this.
Did you change out your axles as well? 28 or 30 spline?

Re: Rear End ID?
 

kept my stock axles...mine went in a 14b sf..

Re: Rear End ID?
 

Try putting some grip tires on the back and do a burn out. I found out the hard way in my young and ignorant years not to do that. The rear went off like a bomb, broke the yoke and carrier housing. Good rear but not for hotrodding.