nelson007

How to identify M20 M21 Muncie transmission

Is there a way to identify a Muncie transmission from the numbers on the side of the transmission?

Thanks in advance

Nelson 007

Easy Mike

They're coded on the cases: A for the M20, B for the M21, and C for the M22.

jnb5101

i don't think there is any reliable way to determine if it's a 20 or 21 by the numbers. wrap tape around the input and output shafts, and scribe a line. then engage first gear and rotate the output shaft a large number of revolutions (say 20 or 30) and count the number of times the input shaft turns. that will give you the ratio of first gear.

nelson007

Thanks for the info. I'm pulling the engine today and transmission. I saw some numbers on the side. Where do I find the Letter that designates what type.
Thanks
Nelson 007

gbvette62

On 66-74 Muncies, an M-20 should have two rings around the input shaft, while M-21's will have a single ring. 63-65 M-20's and 66-74 M-22's, don't have any rings, on the input shaft.

This is a general rule, but not fool proof. I think GM replacement shafts, didn't have any grooves on them, so you may not be able to use the shaft rings, to identify a trans that someone has been into.

When you pull the side cover, you can count the teeth on the input shaft. A 66-74 M-20 will have 21 teeth, M-21's (and M-22's) have 26 teeth. As an FYI, 63-65 M-20's have a 24 tooth input shaft.

Easy Mike

http://www.dandltransmission.com/muncieratio.html

Sunstroked

And that info mike gave you would coincide with my particular Muncie. P2T16A, it is a m20, built in 1972, dec16. At least that's what it was born with.

Faster Rat

...and coincides with my M21 built March 11, 1969

nelson007

Thanks Mike,

I was able to get the date code POC16. This is in a 1970 LT1. I guess I won't know which one it is until I remove it.

Thanks All for the info. I will let you know what I have.

What does the 19s number Ect stand for?

Nelson 007

gbvette62

That's the VIN derivative. "1" is Chevrolet, "9" is Corvette, "S" is St. Louis assembly plant, and the rest of the digits is the car's sequential build number.