Any of you 4 speed experts out there that could help me ID what I have in my 1970. I have the copy of the old build sheet which indicates M-21 close ratio. I did a little research and found that on drivers side of case there should be WB for M-20 or KA for M-21, I could find neither, all I see on the case is General Motors in the casting. I have been told that the 4-speed options were either Muncie M-20, Muncie M-21, or the GM built 4-speed. Is this true? And if so was the GM trans called by any other name or designator. It could be possible that the car was built with M-21 and later someone changed it, not sure, so if any knowledge out there please give me a heads up.

 

go to 442bro.com and click on trans\rear theres some info there, best is to try to get a pic of the side plate with the shift levers, thats a tell all. good luck

 

A picture will not tell all.

The only options you have for 1970 are Muncies M20 wide ratio and M21 close ratio. Often the cases do *not* have the WB/KA identifications on them. And if they don't, there is no way to externally tell a M20 from an M21 (if you can't see the input shaft).
The trans *should* have a VIN derivative vertically on a drivers side rib. And if it matches your VIN, it is the original trans and should be an M21 if that is what is on the build sheet.

 

im thinkin he first wants to id what trans is in there now, muncie , saginaw , super t10, or ?

 

"The only options you have for 1970 are Muncies M20 wide ratio and M21 close ratio. Often the cases do *not* have the WB/KA identifications on them. And if they don't, there is no way to externally tell a M20 from an M21 (if you can't see the input shaft).
The trans *should* have a VIN derivative vertically on a drivers side rib. And if it matches your VIN, it is the original trans and should be an M21 if that is what is on the build sheet."
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Pretty sure the P-code manufacturer's stamp will end in A for M20, B for M21, C for M22- aha, from the 2nd link below:
"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."

This is a pretty good rundown of the various Muncies and how to discern details:
http://www.5speeds.com/muncie2.htm

Here's your side cover pics, casting number tips, etc.
http://www.myss396.com/ChevelleInfo/...s/Muncies.html

Unless the gear ratios are identical, and they are not for M20 vs M21, you can easily discern one from the other by simply counting turns. I do that with every Muncie I get, to verify gear ratios.

For the OP's purposes, yes, start with model and manufacturer ID using side cover shape and bolt count- or handy casting numbers and an internet search. Might as well ck for the car-match VIN derivative stamp. The most obvious clue for different models is whether the case is AL or cast iron.

 

Thanks for the timely correction, Chris. I was thinking the letter suffix started in '72, but I obviously should have looked it up. I know I've seen '69s without it, but who knows why, maybe they were early.

Bottom line is that with all the above info, Bob should be able to get some pretty useful numbers off the trans.