AXLES...

[83cutlasssedan]

On the look out for a new rear end for my Cutlass. See all kinds of things for sale and have a few questions. Don't know much of anything about rear axles so here she goes. What's the difference between 7.5 and 8.5? What exactly is open diff? And whats the difference/advantages between 10 bolt vs 12 bolt etc..
Saw a Dana 35 rear axle with posi for sale? Is that any good...would it fit on the cutlass?

Thanks

 

[axisg]

google is your freind LOL, but for now here are some quick answers:

7.5 vs 8.5
these are the 2 types or rearends offered in G-body cars. 8.5 is stonger and will take more abuse but are pretty rare as they only came in appx >10% of the g-body cars made.

Open Differential vs Posi ( aka limited slip )
open diff ( aka one wheel peel ) rearend will tend to send all the power to the wheel with least traction ( you must have noticed that one wheel will spin on your car when the roads are wet and slick ). Positraction or Limited slip will atttempt to send power to both wheels equally regardless of which wheel has more traction.
Really both rear ends need to have some type of slippage for truning corners as the inner wheel moves much slower than the outer wheel when encountering a turn.

10 bolt vs 12 bolt
kinda the same as the 7.5 vs 8.5 argument but neither will effect you as without a lot of work neither will fit in your g-body.

Dana 35 is typically a truck axle and will not fit your vehicle without a lot of work.

 

[SafariWagon]

Replacements 7.5 should be cheap. 8.5 only in 442 or GN. Does not mean they are all posi. Some 442's are not. And I believe they came with either 3.42 or 3.73 gears. Will throw your speedo off and possibly make the engine rev too much for your liking. A posi 8.5 typically runs $800-$1000. A plain 7.5......... I have one you can have in my backyard if you want to pick it up in Mnpls, Mn

 

[83cutlasssedan]

that's good info thanks a lot.

What size gear are in your 7.5 in your yard. I don't need a replacement just an upgrade.

 

[SafariWagon]

Check this out. viewtopic.php?t=14499&p=109282#p109282 Making a large assumption that a 81 Safari wagon is the same as a 83 Regal wagon I would guess 2.73

 

[megaladon6]

the 7.5/8.5 (and 7.625 of the monte ss) refer to the ring rear diameter. quite simply, larger is stronger. larger ring gears/axles usually have more splines on the axle shafts which is also better.
the 7.5/7.625 is the weakest axle you can get from GM and should not be used over 350hp, but that's not a definitive number.
the 8.5 is MUCH stronger, but g-bodys used a very specific suspension setup and nothing but a g-body rear is bolt in. 8.5s were found in the hurst olds, 442 and GN only. (but IIRC not all H/O or 442's--i want to say 85 and 85 model years)
however there are companies that modify the GM 12 bolt and ford 9in rears. you can also have an 8.5 (or any rear) modified. all they need are the proper mounting points welded on in the correct positions and angles.
10 bolt/12 bolt is just a reference to the number of bolts holding on the diff cover. the 12bolt had an 8.75 ring gear and is very strong, but rare and takes more energy (hp) to drive. same with the ford 9.
they all came in various gear ratios, the only way to know what it is is to turn the input shaft and count how many turns it makes for the axles to make one rev. you can always change the speedo gears in the trans to correct for a different rear.
open diff/posi
a "true"posi is one where there is zero difference in speed/power between the 2 wheels--also known as a locker depending on style. very good for drag launches but sucks on turns and wet roads.
a "positraction" rear (GM's name for it) is actually a Limited Slip Differential. there are many designs but all try to prevent wheel spin by sending the power to both wheels but allow the wheels to turn at different speeds on turns.