www.jdrace.com

Hi Jim,

Dave here. I called and spoke with you a couple weeks ago about installing positraction in my 70 Cutlass. Figured I'd follow up on your forum so you could have some useful info here for other folks looking to do something similar.

So the car is a 70 Cutlass with the original 12 bolt "O" axle, open differential and 2.56 gears. I would like to keep that gear and install a Detroit Truetrac - Torsen style - differential.

It looks like Eaton makes this Truetrac differential for the GM 10 bolt 8.5" axle with 28 spline axles:
1971 - 1989 GM 10 bolt C-Clip 8.5" - 1.20" axle diameter - 28 spline - 2.73 & up gear - Part number 912A556

Are you able to modify this unit to fit the "O" axle? If so, would I buy it and send it to you for modification?

Also, are you planning to do a batch of posi units anytime soon? It might be best if I could get something into your next production run.

No , you can not do any aftermarket unit for a 2.56 or a 2.78 gear in a 12 bolt olds. NO one makes the correct offset. 8.5 offset is for the 3.08 3.23 gear set in the 12 bolt olds conversion. Sorry.
J D Race
Jim Mitschke

Hi Jim.

Thanks for this. I don't mean to push the wrong buttons, but would it be possible to machine some material off the ring gear mounting flange on the carrier or surface grind some material off the back of the ring gear (or a combination of both) to get the proper offset? Or maybe get enough offset so the rest of it can be shimmed out?

I am sure you could... But the support of the flange is critical to defection under loads of the pinion trying to drive the carrier round and round. and the ring gear can only have so much removed also as you have to get the bolts into the ring deep enough for good retention also.

Jim
Then the cost of machining too.