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Any idea which transmission this is? This was in my 1972 LT1. I recently had to pull the motor and thought I would take a pic of the I.D. to see what transmission I have. The only issue is the stamping isn’t very clear. I apologize this is the only pic I got from the mechanic before it got reinstalled. Let me know your thoughts. It’s definately not an “A”. Thank you for your opinions/info.
Raise rear wheels....put car in 1st gear.....put mark on drive shaft...rotate engine exactly 1 revolution...check mark on drive shaft to see how far it rotated....if driveshaft rotated slightly more than 2.5 times around it is a M-20....if it rotated slightly less than 2 and a quarter turns, it is an M-21.
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Originally Posted by stingr69
Raise rear wheels....put car in 1st gear.....put mark on drive shaft...rotate engine exactly 1 revolution...check mark on drive shaft to see how far it rotated....if driveshaft rotated slightly more than 2.5 times around it is a M-20....if it rotated slightly less than 2 and a quarter turns, it is an M-21.
Stingr69 is correct, This is the only way you will know without pulling the tranny out and looking at the input shaft. M21, M22 has a 2.20:1 first gear, M20 has a 2.52:1 first gear. If you have the tranny out, the input shaft has rings, One ring is a M21, 2 rings its a m20 no rings is a M20 or a M22. If it sounds like a box of rocks while driving but it runs great and shifts good and has the correct fluid level, it could be a M22, AKA Rock Crusher, because of the straighter cut gears. The only way to be positive is pull the side plate.
As a bonus, if you arent the original owner and the previous owner has had it rebuilt, it could be any number of things inside there. I just changed my gearing from M21 to M20, it was just the input shaft and the idler gears, easy swap. You can also now buy wide ratio and close ratio aftermarket M22 gears so you have that posibility as well.
Last edited by Rescue Rogers; Jan 12, 2019 at 07:52 AM.
In my limited experience, many who say they have a "Rock Crusher", don't. It is a fairly rare transmission. Like 4-speed C3's being wrapped around trees and plunged into ditches at a far greater rate than the automatics.
Check the 1st gear rotation as ascribed above. M20 is the more likely outcome.
Raise rear wheels....put car in 1st gear.....put mark on drive shaft...rotate engine exactly 1 revolution...check mark on drive shaft to see how far it rotated....if driveshaft rotated slightly more than 2.5 times around it is a M-20....if it rotated slightly less than 2 and a quarter turns, it is an M-21.
I’ve already got her on jack stands since I’m doing a full brake job. I’ll spin the motor a revolution to see how many times the driveshaft spins.
M21 or M22?
