M20 or M21?

[BB wowbagger]

I´ve been investigating my tranny and found out that it is a M20(By the nr on the tranny),but when i check the Corvette Specifications on CAC it says that Close ratio M21 was standard,other sites says tha M20 was standard....I know theres no way to be absolute sure without counting the teeths on the countershaft in the tranny,but does anyone have anymore input on this issue?

 

[mvftw]

You can Not tell what is inside until you open up the side case, your right. But you can tell very EZ what the case is...on the side there are numbers, they will end in a 'A' for M20, 'B'for M21, 'C' for M22. I have a '78 and the code is M21, but that just means that it's a 4sp...I have a ST-10

 

[JohnZ]

The Muncie date code (starts with a "P") stamped vertically in the passenger side of the main case just forward of the joint to the rear bearing retainer will end with an A (M20) B (M21), or C (M22); that will tell what it was originally.

If you have about the same rpm drop between 2nd and 3rd as you do from 3rd to 4th, it's an M21; if the rpm drop from 3rd to 4th is noticeably more than from 2nd to 3rd, it's an M20.

 

[BB wowbagger]

Ok..Then its a M20 ,its really close between the gears but when changing to 4th it feels more like an "overide" gear..Thanks