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M21 or M22?

Old 01-11-2019, 11:00 AM
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brennano1
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Default M21 or M22?

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 isnít very clear. I apologize this is the only pic I got from the mechanic before it got reinstalled. Let me know your thoughts. Itís definately not an ďAĒ. Thank you for your opinions/info.
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Old 01-11-2019, 11:22 AM
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sullyman56
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It looks like the stamp on my 68 M20. P means Muncie followed by K meaning July and then the date. The derivative VIN is from a July 68 Corvette.
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Old 01-11-2019, 11:30 AM
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brennano1
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Itís the last letter that I canít determine. Any idea if itís a bad stamped b or a c? It appears to be lower case.
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Old 01-11-2019, 11:42 AM
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sullyman56
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From what I’ve read 68 Muncies weren’t stamped with a letter after the date code indicating if it was a M20, 21 or 22.
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Old 01-11-2019, 12:08 PM
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Originally Posted by sullyman56 View Post
...From what Iíve read 68 Muncies werenít stamped with a letter after the date code indicating if it was a M20, 21 or 22...
I agree. Looks like the VIN derivative is from a 68.

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Old 01-11-2019, 12:40 PM
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There were only 27004 72's made. This is out of car #27481. I agree--looks like a 68 stamping.
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Old 01-11-2019, 01:27 PM
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stingr69
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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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Old 01-11-2019, 02:05 PM
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That character after the date code is an inspection mark. Sorry, not a "C".
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Old 01-12-2019, 08:48 AM
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Originally Posted by stingr69 View Post
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; 01-12-2019 at 08:52 AM.
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Old 01-12-2019, 09:55 AM
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Don't M22's have a drain plug and 20's and 21's do not?
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Old 01-12-2019, 11:09 AM
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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.
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Old 01-12-2019, 11:12 AM
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no, 70 and newer muncies all have drain plugs, 69 and older do not have drain plugs.. doesn't matter if they are m20, 21, or 22

most people that think they have a factory m22, do not... it is a rare tranny.

i bought a car from a kid once who swore it was a m22 due to the gear whine... i knew it wasn't .. it was dry of oil.. that was the gear whine...
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Old 01-14-2019, 09:29 AM
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Originally Posted by stingr69 View Post
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.
Iíve already got her on jack stands since Iím doing a full brake job. Iíll spin the motor a revolution to see how many times the driveshaft spins.


Thank you everyone for your responses.
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Old 01-14-2019, 11:15 AM
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joewill
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Actually isn't it the other way around?
rotate the engine 2.2 or 2.52 times to get one revolution of the driveshaft... in 1st gear?
my brain hurts...
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Old 01-14-2019, 01:06 PM
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Originally Posted by joewill View Post
Actually isn't it the other way around?
rotate the engine 2.2 or 2.52 times to get one revolution of the driveshaft... in 1st gear?
my brain hurts...
Yep, Mine too.

Good catch - Count ENGINE ROTATIONS per one rotation of the driveshaft. Close ratio M21 Muncie is 2.2:1 ratio and wide ratio M-20 Muncie is 2.52:1

M-22 is close ratio as well.
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