Bozang1

Ok, now that I got everyone's attention, give me your thoughts:
I found a 12 bolt rear the guy claims will ONLY fit a 68-70 cutlass/442. I told him if it fits 68-70 then it should fit upto 72. He said he bought it thinking the same thing but a guy who specializes in rears and works on them for a living said absolutely not. Only 68-70.
??????
Did any cutlass or 442 come with a 12 bolt rear? Is there any reason for this not to fit a 72?
What I'm thinking is that maybe this is out of a different car and he's not saying.
I'm gonna ask him for measurements and go from there.
Any thoughts or factual info that anyone can share regarding this?

Bozang1

I forgot, he said something about it not having the c clips inside to hold the axles. He said the axles are held in by some outside bearing or something. (If any of this makes sense)

Where's the experts at? Give me some info..

70cutty

They are different. 71-72 has a 10 bolt corporate rear end with bolt in axles, parts readily available.
70 and under uses a O type rear end with 12 bolt cover and parts are no longer readily available.The O axle is a good rear,but parts are scarce and expensive. Also it's not a true 12 bolt like chevy 12 bolt, as the ring gear still uses only 10 bolts.

As far as fitting the o type in 72, you will need a longer driveshaft, about 3/4", other than that it will bolt in just fine.

I would just build a 72 rear end, if you have it. It has bolt in axles and it can be build to be just as strong as Chevy 12 bolt.

Bozang1

Maybe I was misinformed or remembering wrong...I believe I was told that one from a 68-72 A body would work.
Bottom line is this: I have the 455, I have a TH400 lined up but it was suggested to me to use a 200R4 or something like that because it's not electronically controlled like the TH400 (I hope you understand what I'm trying to say) and I want to back it up with a posi rear.
I did find a rear with 3.73 gears but it has a spool which probably wouldn't be good for my application (cruising around, every now and then getting on it, not running down the track)
Supposebly, there's something called a Powertrax that I can put in my diff that will make it like a Posi. I'm not sure what gears I have currently though. All I know is it's a 10 bolt rear/open.
I guess it's an 8.5 (I don't know squat about these, never messed with one, is it just a measurement?)
So I think I have a corporate 10 bolt 8.5 open diff and I'll guess the gears are probably like 3.08?
I'm not looking for top end speed, I'd rather have something very quick but with a tall gear/overdrive in the end for cruising between car shows etc.

70cutty

here is my combo, 463cu.in., built up th350, rear end out of 71. Eaton posi, 3.42 gears.
It's quick of the line and it can be driven on the freeway.

Your rear end is just fine ( I am assuming your car is 72 and rear is most likely original to the car) put a posi in it and change the gears, parts are readily available.

200r4 is a 4 speed and needs to be built up to take the abuse from a 455. Even built up, I wouldn't feel comfortable launching with it. In its stock form, it will last about until you step on the gas. There are few guys that build them,I got an estimate from CK performance around $3500 with the converter.

My th350 cost me $700 to build and its taking the abuse just fine. I am also planning to get a gear vendor overdrive unit and bolt it to my th350.

Bozang1

Numerous individuals suggested to me to use a TH400 because that's what comes behind the 455 from the factory and it's made to handle the torque. I am the definition of poor so I'm trying to be cheap but dependable because I do not want to be pulling it out, fixing, installing, pulling it out, fixing, installing...guess I'm just looking for general consensus for the wisest decision so I make the right plan and get the right stuff.
As for the motor, I'd like to just bump up the performance just a little. I'd like to swap out the dishes pistons for flat tops or possibly domed (probably flats) and do a little intake/exhaust upgrades. Not sure if I'd be able to afford head work. I'll look into that when the time comes.

joe_padavano

Your "guy" is misinformed. First, it sounds like the axle you are describing is a Type O axle. Despite the number of bolts holding the cover, it is NOT a "12 bolt" rear end. Remove that cover and you will see only ten bolts holding an 8.5" ring gear. True Chevy 12-bolt axles have 12 bolts holding an 8.875" ring gear. The Type O was unique to Oldsmobile and was used from mid-1966 through 1970. As others have noted, it will bolt up to any A-body, but it was only factory installed from 1966-1970. It does have bolt-in axles, not C-clips, but parts for this unique axle are very difficult to find. Many people buy them for a premium because they think they are 12-bolt axles then get rid of them when the truth comes out. They are plenty strong for most applications, assuming you have the gears you want.

FYI, Oldsmobile A-body cars built in Canada through 1969 DID come with the Chevy 12-bolt axle. These are the only ones.

Fun71

This is incorrect.

Yes, Joe already covered that.
No, there is not. The driveshaft length is slightly different but the rear itself will fit a 68-72 Cutlass (or any A-body) as you stated above.

Koda

If you were to keep the 3.08 gears and put a posi in it, and go with the th-400, it would be a great combo for going down the highway and occasionally getting on it, like you want. Many here insist on an overdrive trans and something like 3.73s with a built 455 when these cars did fine with a 350 and highway gear rear end. A built 455 tied to a 3.08 will launch hard enough on the street to satisfy most people. I am keeping my 442 with its original 3.08 rear, and with a slightly amped up 400, I am sure I will be just fine off the line and be fine blasting at 100, too.

BangScreech4-4-2

I think Koda's on the right track. Probably the simplest thing to do would be to put an Anti-Spin carrier in your existing pumpkin. That rear will likely handle anything your 455 can dish out.

Before you zero in on that though, you may want to determine what ratio you have. Absent documentation, what most people do is jack up the rear (if you don't have Anti-Spin you only have to do one side!), support it firmly and give the elevated wheel a spin for one full revolution while counting the corresponding number of turns for the driveshaft ... approximately three turns = 3.08, 2 3/4 turns = 2.73, 3 1/4 turns = 3.23 and so forth. Some people do ten revolutions for a more exact figure.

Fun71

That will not be accurate with one wheel on the ground due to the differential action causing the driveshaft to turn slower. You would have to multiply the number of times the driveshaft turns by 2 or rotate the wheel two turns instead of one.

BangScreech4-4-2

Did not realize that. Thanks, Kenneth. OP must make sure he supports BOTH rear wheels on jackstands or adjusts his arithmetic, I guess.

joe_padavano

Adjusting the arithmetic is more accurate. The problem is that if the brakes drag at all, there's no guarantee that both wheels will turn the same amount, which screws up the count. Fix one wheel, (hold it or leave it on the ground) and just divide the answer by two. Then you're sure it's correct. Also, it can be difficult to see the difference between 3.23 turns of the driveshaft and 3.42 turns (for example), so I always turn the tire ten times and divide the driveshaft turns by ten. It's much easier to tell the difference between 32.3 turns and 34.2.