Post the casting number that "states it was a M20".

 

Post the casting number that "states it was a M20".

Hint.......there ain't one.

 

Well ,a later one might have the A,B or C code stamped after the Muncie serial number

1969 to 1974 Muncies got a ratio designator at the end of the serial number. An example would be P4D23B. This equates to April 23, 1974, M21 ratio.

But I don't know if that's what he was referring to as this is stamped and not cast as he said.

 

The speed gears are the same on M20's and M21's. You cannot mix a wide ratio cluster with a close ratio input or vice versa. If you've got one ring you've got yourself an M21.

 

660 casting with P9h16a serial number. but this still doesn't answer my question about interchanging input shafts

 

660 casting with P9h16a serial number. but this still doesn't answer my question about interchanging input shafts

In my post I stated that you cannot mix a wide ratio input with a close ratio cluster, or the other way around.

 

The only differences between an M20 gearset and the M21 gearset are the input gear and the cluster, as previously stated. The input and cluster have to be matched. So you can tell the difference by counting the teeth on the input or the cluster gear that engages the input. M21 input = 26 teeth. M20 input = 21 teeth. This is on a 66 to 74 model year Muncie.

Otherwise you can tell a difference by spinning the input while in first gear, and counting the revolutions of the input with respect to the output. The M20 will have 2.52 rotations of the input per one rotation of the output. The M21 will have 2.20 input revolutions per 1 output.

Since you have one ring on the input, expect the 2.20 number.

 

... Can M20 gears be on an M21 input shaft?

Maybe this is the answer you are looking for. Short answer: no. The input shaft and the associated gear is a one piece deal. There is no way to change the gear on the shaft. It probably is not correct to refer to it as a "shaft". It is called the Maindrive.

This is what one looks like.http://www.thegearbox.org/catalog/item/3864782/4902553.htm

 

Thanks guys. Thats what I needed to know. I also apologize MEJ1990TM. I posted my last response before I read your post. I overlooked it somehow. Im hoping to buy an M20 soon so I will use it instead and put this M21 up for a spare. I think an M20 will compliment my 3.43 rear gears much better with a 28 inch tire.

 

No problem, Steve. If you've got a fair amount of torque the close ratio would work with the 3.42's, but if you are running something like the 350 in my car the wide ratio would definitely be better.

I am also on the hunt for an M20. I've got everything I need except a cluster gear. The ST-10 in the car has just a little more gear at a 2.64 first. But the Muncie should be a nicer shifting transmission, it'll be a little stronger than the ST, and it also won't be so noisy.

 

forget all the #'s, like said, put it in first, see how many times the input turns to the output
quicker than typing on here, lol