StormAuto

I've never been much of an engine guy, I've spent most of my life as a paint and body guy so pardon my ignorance. My 70 Cutlass has a 12 bolt rear end, is a 12 bolt better than a 10 bolt?

redoldsman

That is what is referred to as an "O" type rear. It has a 12 bolt cover but the ring gear is only 10 bolts.

StormAuto

Thanks for the info!

wmachine

Whether it is better or not also depends on what carrier you have, but *generally* speaking, your O rear is better than the the previous 10 bolt P rears, but not quite as good as and the subsequent 10 bolt "corporate" rears.
Not to be confused with the Chevy (true) 12 bolts that are stronger than all of them.

joe_padavano

To expand on Kurt's post, people frequently confuse the Type O for a Chevy 12 bolt and assume it is just as strong (and worth just as much). Most folks assume that 10-bolt and 12-bolt refer to the bolts on the cover. They actually refer to the bolts on the ring gear. The Chevy axles coincidentally have the same number of cover bolts as ring gear bolts, but other GM axles do not. The Chevy 12 bolt axle has twelve has an 8.875" ring gear held on with 12 bolts. The Chevy 10 bolt axle has an 8.5" ring gear held on with 10 bolts. The Type O also has an 8.5" ring gear held on with ten bolts, however the parts do not interchange with those of the Chevy axle.

StormAuto

Thanks!!!!!

Run to Rund

To expand on the above correct information, the 12 bolt C uses 3/8" ring gear bolts while the O uses 7/16" bolts so whether there are 10 or 12 of them isn't important. Other design features are important. The 68 type O used 31 spline axles but 69-70 used 28 spline. 12 bolt C are 30 spline, but with the c clip design which isn't good but upgradeable with aftermarket parts to positive axle retention with sealed bearings. The o and late 10 bolts are 8.5" ring gear, while the 12 bolt c has 8.87" ring gear.