I have a 72 Cutlass. Bought this years ago. The guy said it was a posi out of a chevelle and would fit in my car. He said it has highway gears so I'm wanting to change that to a 3.73 if possible. There is an F stamped into the case where the gasket goes for the cover. On the gears it stamped with:
GM 554278 15:41 78

Anyone know what this means?

 

It's a 2.73:1 ratio. I have read in other posts that certain carriers have limits on what ratio gears will work. you may need to get the casting number off the carrier.
Also looking at the welds it looks like you have a poor mans posi. Not a good idea, IMO.
Here is a link that has some info.
http://www.supercarsunlimited.com/in...id/diff_id.htm

 

looks like a 7.5" rearend that has been welded.
Heap o junk.

 

This don't sound good. I think the # on the carrier was 912 unless I'm not looking at the right #.

 

Throw the whole thing in the dumpster or in the pile to go to the scrap metal dealer! Its probably not worthy of a 'boat anchor"!!

 

That's a 7.5" rear,like what came out of the G-body cars,and that is not a posi anyway.That is an open carrier that someone welded the spider gears,to lock everything together.Just walk away.There is nothing right about it.

 

Hi can someone point out the weld locations in these pics so I can understand thanks

 

Would the axles fit in a 8.5 inch?

 

No. 7.5" have 26 splines and 8.5 have 28 or 30

 

http://wichita.craigslist.org/pts/2525299723.html

Any idea if this would fit? Or could I take out the guts and put them in my 10 bolt open rearend that's under the car?

 

Well, some of it would fit.

That craigslist rear end is from a leaf spring car/truck. There are no coil spring perches on the housing.

Your stock 72 rear is 55.5" in between the backing plates, and your bolt pattern is 5 x 4&3/4
He said 58.5" so the axles are too long. I'd hope he's just mistaken on the bolt pattern being 4.5"

You could use the guts, and the brakes, but not the housing.
The axles are iffy. You would most likely have to re use your stock axles.

 

If you look at the spider gears you will see that one of the tooth valleys has been filled in with a welder. This does not allow for a differential action. The heat weakens the metal, and the car will lurch when turning a tight corner.

 

Why don't you hook up with Brian Trick (507 Olds) or Jim Mitschke (Monzaz) to work out something for your car. Both are excellent rear end specialists (be kind) and sell great products. They both KNOW what will work in your car and what won't. Don't guess, send a PM and ask.

PS, don't base your differential performance on the lowest price. Posi units (especially the new Auburn and Eatons) don't come all that cheap, and you need the right ring/pinion to make your car work. Getting a Series 2 or Series 3 GM carrier is harder and harder nowadays. And if you're planning to use the posi for some heart pounding asphalt burning, go with the new carriers - they're designed for the 'abuse'. BTW your axle count in 1972 is 28 spline and your carrier was originally a 10 bolt corporate. Lots of people don't know what 10 bolt means - it's the number of bolts that secure the ring gear to the posi. Don't be fooled by looking at the number of bolts on your diff cover.

You will find out this and more by talking with one of the guys I mentioned above. Good luck in your quest.