I just went out to look at what trans was in my car turns out its a m-20. IM running a 1972 454 bored .60 over its stock hp was 270 but i think its 350 or around ther with the 4150 holly and holly manifold. My question is what rear end would you suggest to go with the car it currently has a 2.53 or somthing non posi 10 bolt. im thinking about a 12 bolt 3.08 or 3.55 im no trying to build a drag car but just somthing to cruise around in that has pickup when you hit the gas. Thanks Tom

You could run a 10 bolt, but remember you have alot of torque with a 454. Through a manual trans all that torque is applied right to the rear end. If you have access to a 12 bolt, I think it would be the way to go. Depending on your tire size/height a 355 gear isn't out of line.
Just my opinion, Rob

is the 12 bolt heavier?

The 12 bolt is a little heavier, but stronger. It would be a good idea with a 4 speed. If you had a automatic I would say that the 10 bolt would probably hold up fine. If you decide to stay with the 10 bolt then you should get a new Eaton posi (stay away from Auburn) and have it set up by a pro. There are some other things that can be done to make it stronger.

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'69 RS/SS396 Pro Street
427 4 speed 9"
Byars Performance
High Performance Drivetrain Parts & Service
www.lubedealer.com/biggearhead (http://www.lubedealer.com/biggearhead)

thanks guys this is his son

In time, chances are very good that the 454 will break the 10bolt unless you granny it all the time. Your 2.56 will help with fuel economy, and, be great on the hiway. With the torque which you will produce with the 454, a 3.31 or a 3.08 (3.07 in a 12bolt) will respond just fine. My 70 has a SB400, M-20 and 3.07 12bolt posi and it will break loose the 2556015 tires with little effort.

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Tom Parsons

Also will the trans hold up to the 454 im not sure what the hp rating is or tourqe rating all i can see is it has the 4150 holly with the holly intake cylinders are reading about 10.1 10 1/2.1 will the m-20 hold it? I was reading in the older forums theres no real big dif between the m-20 and m-22 strength just differnt first gear.

IMHO it'll hold it fine UNLESS you put big stickes on and start dropping the clutch at 4500. Since you are buying gears, and I run a 3.55 rear, tis just a mite steep for highway IMHO. 3.31s or 3.08/7 would be a nicer cruising gear.

My '72 SS502 is equipped with a 3.31:1 cogged 12-bolt posi, an M-20 and a set of BFG P275/60-15 tires (28" tall). Retina-seperating gobs of giddy up down low, and reasonable RPM's while cruising the highway at 65-75 MPH. The 3.31's are a great "middle of the road" gear choice, IMHO.

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Gregg Haskin
72 Chevelle SS
ZZ502 Crated RAT
Muncie M-20 4 speed
TEAM CHEVELLE #726 ACES #4486
"PROUD TO BE AN AMERICAN!"
“What the heck are all these extra nuts & bolts for?”

My 72 Chevelle SS Restoration Website: www.72ChevelleSS.com (http://www.72ChevelleSS.com)

Thanks a ton. Its a big choice