I have a 71 cutlass S with a 2 bbl 350/ th350. I have plans on dropping a 455 with 450+ hp and Im thinking 700r4 in the car. But that is a year or 2 down the road. this is going to be a street car for the most part but may race it a little. my nest project is the rear end, and I could use some help. I have a BOP corporate 10 bolt 8.5 inch open with 2.73 gears I think. It has the R2 code. What Im thinking is a posi with 3.42 gears. I don't want to spend a fortune on this and plan on doing most of the work my self. Some things I would like to know is what is the difference between the carrier with the S spring in it and the one with the 2 plates and a bunch of little springs. Should I get new axels and if so where and what do I look for. any help you all could help with would be great.

 

It's clear that at the least Brian Trick and Monzaz are expert differential builders. I have basically have the same car as you in a 69, went to buy a complete 3.42 and it ended-up being a 3.23 upon inspection. I think it was a fair price so I followed through with the purchase anyway. Looking at a calculator posted by oldcutlass I can tell you that with stock diameter tires you will be turning your 350 at about 3300 at 70mph.

 

I can live with the RPM for now. I only want to do this once.

 

If you stay with a 3-spd trans,go with the 3:42. If you are going with an overdrive,go with the 3:73.
The posi units with the plates,and 4 coil springs,is an Eaton or Yukon.They are both clutch style units,and rebuildable. The style with the S-spring is the original GM clutch style units.

 

that clears up a lot for me, thanks. am I correct that I can use any gm 8.5 posi carrier? and I have been looking at axles shafts and all the axles shafts I can find seem to be for C clip. my next question is can I go to 30 spline or should I stay with 28 spline?

 

All axle shafts for the 10 bolt 8.5 will be custom built. No direct bolt in off the shelf axles fir the 71 72 skylark / cutlass.

If you are trying to watch your money just re-bearing the rear install the posi and gears. (28 spline stock axles will be fine for as long as it sounds that it will take you to get funds for a 455 motor. and 700r4 trans. BUT I could be wrong if you win the lottery. Jim

 

wow no love for olds is there. thanks for the info monzaz. looks like that's what I will be doing.

 

I have the same rear and it is 2.56 gears and not 2.73. Doesn't really matter if you're switching to a LSD carrier, though.

197110boltR2256opendiffcomp.jpg

 

Not all GM 8.5 posi carriers will work. Corvettes have 17 spline carriers in the mid 70's. I bought one thinking it would work. Also there are the 2 series and 3 series. The 2 series wont work with a 3.-- and numerically higher gear unless you have a spacer for it.