Conversions & Hybrids - What Differential are you using.

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05-05-2012, 04:31 PM
What differential are you using for your swap? Alot of you guys probably have a stock posi in your car but mine is an open differential and need to upgrade. I have a '72 Cutlass S with a 10 bolt cover, 10 bolt ring. Should I just upgrade the differential gear or do you think I should swap the full rear end assembly with one from a 442 or Chevelle?

05-05-2012, 04:52 PM
Your Cutlass rear is a very strong 8.5" 10 bolt with 28 spline axles. You do not need to change your rear end assembly. Many people use these rears (71-72 cutlass and skylark) in earlier A-bodies because they are stronger then their 8.2" and they bolt right in. So keep the rear you have.

To upgrade you should go with an Eaton Posi unit if you are mostly a street cruiser. Don't do a detroit locker on the street.

And if you wanted to change the rear gear ratio you should go with Motive for a ring and pinion gear. Timkin bearing kit would also be a good thing to have installed while its apart.

If you need a source for these parts let me know, there are some well known Olds guys that build and supply parts for these and can get you set up.

05-05-2012, 04:54 PM
Depend on how much HP your going to be sending through it.?

The 72 was a chage over year. Some have the 8.5 and other the 8.2, Hopefully you have a 10 Bolt 8.5" rear vs. the smaller 8.2".

I know people will tell you to go bigger with a 12 Bolt or 9"/8.8" Ford. But many racers in my area will tell you that an correctly built 8.5 will go in to the 9 sec and hold up well.

I have a 12 bolt in my 71' Chevelle that I swapped in and it had a 8.5. But for my Nova, it had a 8.2 and 12 bolts are very hard to find at a good price. I am going to build an 8.5 with a Truetrac or Eaton center, some good gears and 30 spline HD Axles. That should be good enough for my build.

So, if you can find a good 12 bolt A-Body housing to build up that would be a good start(they are easier to find than a left spring one), But if not the 8.5 will be good enough and will be more budget friendly for a mild built engine.

just my 2 cents.


05-05-2012, 04:54 PM
Also to make sure you still have the factory rear end you can check these couple of things.

The nut on the pinion for the rear end will use a 1 1/4" socket.

The rear u-joint which attaches to the pinion will use 4 bolts with 2 straps and 4 nuts. Not the typical 2 u-bolt type that the smaller 8.2" rears use.

05-05-2012, 06:38 PM
I have a TruTrac in my stuff..

05-05-2012, 08:21 PM
Merv, What is your review of the TruTrac ??.. From talking to some, That is the direction I'm heading for now, unless I get talked out of it. ???

Does it hold up will in Autocrossing or in the road and around two for a DD ?



05-05-2012, 10:20 PM
I'm happy with it.. Doesn't slip, and kinda is torque-proportioning by design.

Run one in the car and in the S10.:)

05-05-2012, 10:36 PM
I was looking at the TrueTrac and Eaton Posi. TrueTrac looks nice and more appealing because you don't have to rebuild them like others. Will they still work with my drop out axles since I don't have C Clip axles?

05-06-2012, 02:44 PM
I have no problems with the one in my 9"..the 7.5" Eaton I have is a C-clip rear, but the way they are made the Spider Gears stay inplace..the blocks that are in the center are to keep the axles apart and keep the C-clips in. I don't see and problems if you ran it without clips.