69 Cutlass S rear differential question
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Teching In
Joined: Oct 2012
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From: West MI
69 Cutlass S rear differential question We're doing a 6.2L w/ a 4L65 trans swap in a 69 Cutlass S. We have the engine/trans installed, all the wiring done, and we did Hotchkis trailing arms.
The car is nearing completion; I actually drove it Monday!!
One of the things left to do is the rear differential. I don't know much at all about rear diffs. The original plan was to convert the one wheel peel rear diff that's in it (not sure what ratio it is) over to a 3:73 w/ a locker.
The issue I've run into is this; the guy doing the rear diff work told me the rear diff is basically a 12 bolt cover w/ a 10 bolt ring and pinion; and something GM only used for 3 years. He said he can't find parts for it and the price went from around $1200 up to $3k because he wants to use either a Ford 9" or a GM 12 bolt (the $3k included doing a disc brake conversion the the rear end).
I'm not at all saying he's trying to rip me off, I just wanted to post here to see if not using the rear diff that's in it is the right thing to do. Has anyone here face a similar scenario; and if so, what did you do, or what would you do if you were in the same situation?
I don't entirely care about the disc brake conversion for now. But having a posi w/ a 3:73 is kind of a must for this car. The diff that's in it is lacking at best.
The car is nearing completion; I actually drove it Monday!!
One of the things left to do is the rear differential. I don't know much at all about rear diffs. The original plan was to convert the one wheel peel rear diff that's in it (not sure what ratio it is) over to a 3:73 w/ a locker.
The issue I've run into is this; the guy doing the rear diff work told me the rear diff is basically a 12 bolt cover w/ a 10 bolt ring and pinion; and something GM only used for 3 years. He said he can't find parts for it and the price went from around $1200 up to $3k because he wants to use either a Ford 9" or a GM 12 bolt (the $3k included doing a disc brake conversion the the rear end).
I'm not at all saying he's trying to rip me off, I just wanted to post here to see if not using the rear diff that's in it is the right thing to do. Has anyone here face a similar scenario; and if so, what did you do, or what would you do if you were in the same situation?
I don't entirely care about the disc brake conversion for now. But having a posi w/ a 3:73 is kind of a must for this car. The diff that's in it is lacking at best.
The hot set up is to put in a rear end out of a 1971 Cutlass. They are the stronger 8.5" rear and, if it is like Buicks, it will have bolt in axles which are safer. The 68, 69 & 70 rears are typically weak 8.2" models. Here is the guy I use for my rear end stuff. He is an old school muscle car specialist and a good guy.
Jim Mitschke:
J D Race
3641 Southern Road
Richfield, oh 44286
http://jdrace.com
330-990-8155
Jim Mitschke:
J D Race
3641 Southern Road
Richfield, oh 44286
http://jdrace.com
330-990-8155
The BOP rear ends tend to be rarer than the Chevy version. You might consider looking for a complete chevelle rear end with the correct ratio. A 12 bolt is nice, but if you're not planning on racing, a 10 bolt would be fine and a lot cheaper.
I'm running a 10 bolt in my 72. If you swap in a chevy, be sure to correct your pinion angle and check driveshaft length.
I'm running a 10 bolt in my 72. If you swap in a chevy, be sure to correct your pinion angle and check driveshaft length.
Thread Starter
Teching In
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From: West MI
We're leaning towards the full monte at this point. Still trying to decide between Ford 9" or GM 12 bolt, but we are swapping out the goofy rear diff that's in it, and doing the disc brake conversion.
Thread Starter
Teching In
Joined: Oct 2012
Posts: 31
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From: West MI
Here's a couple pics. Sorry for the quality, they were taken w/ my cell phone. I actually started a thread a while back about this conversion; once this is done (are they ever really done?!) I'll update that w/ some better pics.
We also did Foose Legend 18" wheels.
The car
The engine
We also did Foose Legend 18" wheels.
The car
The engine
Last edited by mattsk8; Apr 4, 2013 at 12:16 PM.
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Joined: Dec 2005
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The hot set up is to put in a rear end out of a 1971 Cutlass. They are the stronger 8.5" rear and, if it is like Buicks, it will have bolt in axles which are safer. The 68, 69 & 70 rears are typically weak 8.2" models. Here is the guy I use for my rear end stuff. He is an old school muscle car specialist and a good guy.
Jim Mitschke:
J D Race
3641 Southern Road
Richfield, oh 44286
http://jdrace.com
330-990-8155
Jim Mitschke:
J D Race
3641 Southern Road
Richfield, oh 44286
http://jdrace.com
330-990-8155
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Teching In
Joined: Sep 2012
Posts: 3
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From: Wakefield, Massachusetts
The rear axle you have is known as the type "O" 12 bolt which, as you found, is a Oldsmobile made 12 bolt housing with a 10 bolt ring gear of just slightly under 8.5". Parts are available from Supercars Unlimited as well as Gevo's rears and gears and probably a few others as well. Gevo was getting around $450 for a rebuilt posi differential last I saw and he should have gears too. If he doesn't Supercars will. You can get 3.42's or 3.91's but not 3.73's so if your heart is set on that ratio you'll need to replace the entire axle. Putting rear disc on that axle is a little tricky because it has bolt in axle shafts that will require a shim between the axle bearing and retainer. I've done it and while not super difficult it's time consuming. Good luck!
Tony
69 442 LQ4
Tony
69 442 LQ4
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The BOP rear ends tend to be rarer than the Chevy version. You might consider looking for a complete chevelle rear end with the correct ratio. A 12 bolt is nice, but if you're not planning on racing, a 10 bolt would be fine and a lot cheaper.
I'm running a 10 bolt in my 72. If you swap in a chevy, be sure to correct your pinion angle and check driveshaft length.
I'm running a 10 bolt in my 72. If you swap in a chevy, be sure to correct your pinion angle and check driveshaft length.
Mine's a 72 with 1" seback mounts connected to a Muncie and Olds trans crossmember. Engine was 3 degrees down. Stock Olds upper control arms result was flat, 0 degrees. Got some adjustable upper control arms, adjusted 3 degrees up, problem solved.
Also, the GM 10 bolt pinion is about 1/2" farther back that the Olds. /that combined with the trans being 1/4 to 1/2" forward, and an oddball driveshaft with only a 5" yoke meant I needed a new driveshaft.
Pic of UCA below
Also, the GM 10 bolt pinion is about 1/2" farther back that the Olds. /that combined with the trans being 1/4 to 1/2" forward, and an oddball driveshaft with only a 5" yoke meant I needed a new driveshaft.
Pic of UCA below
Last edited by garys 68; Apr 5, 2013 at 07:32 AM.
