Transmission ID

[michael l. oakley]

I have 1969 convertable 4 speed car. How can I identify the transmission as one of the M series units, M-20, M-21 or M-22...?

 

[JohnZ]

I have 1969 convertable 4 speed car. How can I identify the transmission as one of the M series units, M-20, M-21 or M-22...?

All '64-'74 4-speed Corvettes used the Muncie 4-speed transmission. It was available in three flavors on the order blank - M20 with wide-ratio gears, M21 with close-ratio gears, and the heavy-duty M22 with close-ratio gears (only on certain engines).

If the rpm drop between 3rd and 4th is significantly more than from 1-2 or 2-3, it's an M20 wide-ratio; if the 3-4 rpm drop is about the same as 1-2 and 2-3, it's the M21 close-ratio. If your tank sticker is still on top of the fuel tank, it will show which version the car was built with. There's no way to tell externally which one it is - they all used the same cases - only the internal gearsets were different.

 

[Tom73]

John,
You mean that there is no stamping/part # on the case that would id the trans? Seems like that would have been a real problem on the assembly line.

 

[Crimson Thunder]

I have to do this from memory so bear with me.On the pass side of your transmission going up and down should be some numbers.One is your vin number.The other is a date code.In the date code there will be an A,B,or C.The A=M20 B=M21 C=M22.Sorry i couldn't be more precise but all my reference material is still packed away.

 

[JohnZ]

Oops! Forgot we're talking about a '69 - I'm rooted in the midyear era. If the transmission was built at Muncie after mid-October, 1968, the Muncie "P"-code stamped vertically on the rib at the rear of the main case on the passenger side will tell you which gearset it has; if the suffix is "A", it's an M20. If it's a "B", it's an M21. If it's "C", it's an M22 (highly unlikely).

There was also a steel tag under the lower forward side cover bolt embossed with the transmission assembly part number (which is how the assembly plant identified it), but most of those disappeared the first time the transmission was rebuilt.

 

[michael l. oakley]

After the P code it has 12-22 stamped on it.

 

[JohnZ]

After the P code it has 12-22 stamped on it.

If there's nothing after the "1222", it's a 1966 or earlier Muncie; what's the casting number on the passenger side of the main case? It will start with 385, 388, or 392.