Sarum

Hi from New Zealand, I have an original 10 bolt Positrac (?) diff in my much loved '65 Cutlass Sports Coupe as well as a shiftkitted TH350 trans.I know the limited slip still works because while trying to get my car up onto ramps in my garage I ended up with 2 nice 225mm black stripes on the concrete floor!The engine is a somewhat modded 315bhp (originally) 330 V8.
The question is " Do I risk damaging the diff (probably a 3.08) during take off when doing a small amount of drag racing?" I only race the car a couple of times a year with up to 10 runs at a time.
Also I am thinking of putting a 2400rpm high stall torque converter in the car because of the high lift cam installed to sidestep the bogging issue.Thoughts?

jpc647

Someone on the forum may correct me, but Oldsmobile 12 bolt rear ends aren't much(if at all) stronger than the 10 bolt. Yes, they may have 12 bolts on the cover, but the ring gear isn't any bigger than in a 10 bolt rear. A 12 bolt olds rear is not like a 12 bolt chevy rear, it's nowhere near as strong.

I remember reading a thread on I think realoldspower.com before the site crashed, and it was very informative.

I think the 10 bolt rear will be just fine for the drag racing you will be doing. It'd just make sure the fluid is still good, and it's full, and the proper posi additive is in there too.

Smitty275

With the 330 you'll be fine.

RandyS

I beat the snot out of the rear in my 66 442, and the worst that ever happened was I broke an axle.

oldcutlass

Drive it like you stole it.

Run to Rund

The 1971-2 A body with the 8.5" ring geared ten bolt was a good bit stronger than the early 8.2" ring gear diff. I believe the above comparison was made to the 8.5 not the 8.2. I broke spiders in it with slicks and a 425, a lot more than the 330 stock engine puts out. As Smitty said, you are probably OK.

joe_padavano

This is the real issue. You're not going to hurt the center section unless you can get the power to the ground. Unless you are running slicks and launching hard, the tires will spin before any damage can be done.

Fun71

jpc647,

As Run to Rund said above, the '65 would have an 8.2" 10 bolt, which is not the same thing as the later ('71-up) 8.5" 10 bolt.

RandyS

I should have been more clear on my post. I was just trying to let the OP know that I couldn't break an 8.2, and I tried my damndest..........

jpc647

You're absolutely right. Sorry.

JohnnyBs68S

I was always curious about this ("O" axle vs. "real" 12-bolt rear end strength), what is it about the "real" (8.5") 12-bolt rear end that makes it stronger than the Olds "O" (8.5") 10-bolt rear end? Two more bolts? Has anyone snapped off 10 ring gear bolts by putting too much torque through it? Or does the "real" 12-bolt rear use a beefier housing / bearings / axles?

I do realize that not all "O" axles were created the same, some had stronger axles from the factory w/ higher spline counts.

EDIT: I may have found the answer in an adjacent thread:

Fun71

The only broken O-Type 12 bolt I ever saw (back in the '80s) had a cracked carrier. That was a 3.91 rear behind a '70 W-30 manual trans car that saw hard launches at the track.

Run to Rund

The O uses 7/16" bolts while the Type C 12 bolt uses 3/8" bolts; clearly mounting bolt strength is not an issue. The C has a good reputation based partly on stock stuff like the housing, but also largely on aftermarket posi, axles, pro gears, etc. that are available for it.