Adding Posi

[HurstandGN]

Hey Guys, I am restoring my 84 H/O that did not come with a posi rear as that was an options only for that year. I have the 8.5 rear with 3:73 gears. My question is, can a posi system be added to it with out to much trouble, and what would that process be.
Thanks,

Paul

 

[pontiacgp]

If you can find a 3 series 8.5 posi carrier it's about a 2-3 hour job to switch out carriers...if you can get your car on a lift less than 2 hours

 

[HurstandGN]

So it's a different center section altogether?? I have the frame out and most of the way apart, the rear will be off tomorrow.

 

[pontiacgp]

the "center" section aka carrier is where the posi is. You have an open carrier now. The backlash shims (shims on either side of the carrier) that are in the rear end now should give the posi carrier backlash that's within the tolerance of .006 to .010 so you want to make sure the shims go back in the way thy came out

 

[HurstandGN]

Thanks for the help, the search is on.

 

[motorholmes]

I picked up a GM posi unit out of an S-10 for my 7.5" and had a buddy that works at a transmission shop put it in. I've been told that it can be tricky to set the pinion bearing preload and the lash correctly, so you might want to either get some help or pay someone. I got mine put in for $80 (which is very cheap) not including new seals/bearings.

Just to clarify, the "center section" is the exterior housing that your axle tubes are stuck into. The posi unit you're looking for will fit in your hand and it bolts up to the ring gear in your rear end. The stock GM units are supposedly very good, if you can grab one for $100 or less.

 

[pontiacgp]

if you're just changing the carrier the pinion is not removed and there is no reason to touch it. Changing the carrier does not affect the pattern of the gears meshing. The only adjustment that may be needed is the backlash

 

[motorholmes]

if you're just changing the carrier the pinion is not removed and there is no reason to touch it. Changing the carrier does not affect the pattern of the gears meshing. The only adjustment that may be needed is the backlash

^^ Er, ummm, Yeah what he said! Sorry, I did a gear swap also. Of course you don't have to worry about that 😳

 

[pontiacgp]

you made a good point to make him aware of what's required to set up the pinion so that it isn't touched unless it's needed. You got a great deal on setting up the rear end for $80.

 

[Pontiac455]

No carrier are ever cast the exact same so a check of the gear meash is a must. Anytime I change the carrier I always check the mesh pattern, its kinda late when its all back together and the gears are noisy. Better safe than sorry.