Hello;

I have a '70 Chevelle originally L65 (350 2bbl) with TH400 Tranny. I would like to know if non-SS cars could be ordered with a 12 bolt rear. I know that the build sheet shows rear gear ratio and whether or not it is posi, but where would it say if it is a 10 bolt or 12 bolt (if this was even possible)

I would like to find out whether this car had a 12 bolt originally (not original rear currently in car.) I do have the build sheet.

Your help is appreciated,

Thanks, Kyle

mine came with the 12 bolt with 331 gears. mine is not an ss but came also with round gauges, buckets, 4 speed muncie m20.

Pitt;

If you have the build sheet, is there anything on there denoting the fact that a 12 bolt was installed?

All of the big block cars had additional frame braces installed between the upper and lower controll arm mounts on the frame. I would guess that any car with the 12 bolt would have these braces installed also. I know that the Monte Carlo 12 bolt rear ends that I have removed had these braces with the 350 engines and automatic transmissions.

Pretty sure it is Malibus with 4 speeds (Small or Big), or big blocks

Okay;

This car does have additional braces in the rear along with boxed rear control arms and a rear sway bar. Of course, I have no way of knowing if these were swapped in by a previous owner or not. Current rear end is a 10 bolt out of I believe a 71 chevelle. My build sheet does not note any F41 suspension (was that what rear sway bar was included in?).

Is there no real way of knowing if this car originally had a 12 bolt or not?

Pretty sure it is Malibus with 4 speeds (Small or Big), or big blocks
This is absolutely true through 67, the 68 and up cars I believe it became a standard thing in BB cars.

Build sheet would show F-41, special suspension, and also axle ratio. A 3.36 and 3.08 is a 10 bolt, 3.31 and 3.07 is a 12 bolt. If it is 2.73, they were available in both.

My 72 Malibu came with a 12 bolt, 402 big block, turbo 400 tranny with column shift and a bench seat.

1970 through 1972, RPO code YD1 - Trailering Axle. This option was the h.d. 12-bolt axle with the 3.31 ratio. It was available on any V8 Chevelle with the Turbo-hydramatic and F40 h.d. suspension. It was offered all the way down to the 307. The 350 4-barrel Chevelles in that period already used the 12-bolt rear but with taller gearing, typically 2.73 or 2.56:1.

As the name suggests, this option was for trailering. The General required you to specify the 3-speed automatic & h.d. suspension to that end.



The 307 & 350 2-barrel 1970 Chevelles came with 3.36 10-bolts when ordered with 4-speed transmissions. The 350 4-barrel & 402 already came with the 3.31 12-bolt when paired with the 4-speed.

my build sheet does not say 12 bolt anywhere,it says g94 331 ratio axle

My 70 was a plain jane 350/350 car that came with a 12 bolt and 331 gears. I did not have any additional bracing at the rear control arms. That is how the car was setup when I got it and I have no reason to believe that anything had been changed since it left the factory.

In fact, my gear ratio was originally posi with 2.56:1. I've never seen rpo YD1 on buildsheet before....

What's the date/ratio code on the rearend? Stamped on Pass side forward face of the axle tube, offcenter-closer to the backing plate.
A 3.31 ratio noted on the build sheet would indicate a 12 bolt. As Doug mentioned, a 10 bolt would have a 3.36 ratio.