Differences in M20 & M21

[Tom Freeman]

How can you tell the differences in the M20 and M21 4-speeds? What are the visual differences?

[Joe L.]

Re: Differences in M20 & M21

Tom----

IF the transmission input shaft is original, you can tell by observing it. ORIGINAL Muncie input shafts have annular grooves to identify gear ratio. Two annular grooves indicates a wide ratio(M-20); 1 annular groove indicates a close ratio (M-21); NO annular grooves indicates a close ratio, heavy duty(M-22).

Bear in mind that many replacement input shafts, including all General Motors "second issue" shafts do not have the annular grooves. So, if you find a transmission with NO grooves it doesn't necessarily mean that it's an M-22.

If you cannot determine the gear ratio by the grooves, then you can do one of two things: (1) remove the side cover and count the teeth on the input shaft gear. 63-65 M-20 has 24 teeth; 66-74 M-20 has 21 teeth; 63-74 M-21 and M-22 have 26 teeth. (2)or, with the transmission in 1st gear, you can rotate the input shaft and observe the number of rotations of the output shaft. The relationship represents the 1st gear ratio.

You CANNOT tell an M-20 from an M-21 (or M-22)by course or fine spline input shafts. ALL 63-70 Corvette Muncies used 10 spline inputs and 26 spline outputs. ALL 71-74 Muncies used 26 spline inputs and 32 spline outputs.

[Joe L.]

Re: Differences in M20 & M21

Tom----

IF the transmission input shaft is original, you can tell by observing it. ORIGINAL Muncie input shafts have annular grooves to identify gear ratio. Two annular grooves indicates a wide ratio(M-20); 1 annular groove indicates a close ratio (M-21); NO annular grooves indicates a close ratio, heavy duty(M-22).

Bear in mind that many replacement input shafts, including all General Motors "second issue" shafts do not have the annular grooves. So, if you find a transmission with NO grooves it doesn't necessarily mean that it's an M-22.

If you cannot determine the gear ratio by the grooves, then you can do one of two things: (1) remove the side cover and count the teeth on the input shaft gear. 63-65 M-20 has 24 teeth; 66-74 M-20 has 21 teeth; 63-74 M-21 and M-22 have 26 teeth. (2)or, with the transmission in 1st gear, you can rotate the input shaft and observe the number of rotations of the output shaft. The relationship represents the 1st gear ratio.

You CANNOT tell an M-20 from an M-21 (or M-22)by course or fine spline input shafts. ALL 63-70 Corvette Muncies used 10 spline inputs and 26 spline outputs. ALL 71-74 Muncies used 26 spline inputs and 32 spline outputs.

[Tom Freeman]

Thank you Joe,...

Good to see that you are on duty tonight. So basicly the cases are the same, right?

[Tom Freeman]

Thank you Joe,...

Good to see that you are on duty tonight. So basicly the cases are the same, right?

[Joe L.]

Re: Thank you Joe,...

Tom----

Yes, the cases are basically the same, but there are some differences:

63 main case #3831704 M-20 and M-21-----------7/8" countergear shaft

64-65 main case #3851325 M-20 and M-21---------7/8" countergear shaft

66-67 main case #3885010 M-20, M-21, M-22-----1" countergear shaft

68-69 main case #3925660 M-20, M-21, M-22------1" countergear shaft

70-74 main case #3925661 M-20, M-21, M-22------1" countergear shaft

The 63-65 main cases are basically the same; the 66-74 main cases are basically the same. M-22 main cases and many 3925661 main cases are equipped with a drain plug which is fitted to the lower boss present on all Muncie maincases.

63-70 use three different tail shaft housings GM # 3831731, 3846429, and 3857584. All are basically the same. 1971-74 use tailshaft housing GM #3978764.

[Joe L.]

Re: Thank you Joe,...

Tom----

Yes, the cases are basically the same, but there are some differences:

63 main case #3831704 M-20 and M-21-----------7/8" countergear shaft

64-65 main case #3851325 M-20 and M-21---------7/8" countergear shaft

66-67 main case #3885010 M-20, M-21, M-22-----1" countergear shaft

68-69 main case #3925660 M-20, M-21, M-22------1" countergear shaft

70-74 main case #3925661 M-20, M-21, M-22------1" countergear shaft

The 63-65 main cases are basically the same; the 66-74 main cases are basically the same. M-22 main cases and many 3925661 main cases are equipped with a drain plug which is fitted to the lower boss present on all Muncie maincases.

63-70 use three different tail shaft housings GM # 3831731, 3846429, and 3857584. All are basically the same. 1971-74 use tailshaft housing GM #3978764.

[Bill Clupper]

Re: Thank you Joe,...

You might also note that in the early case M-20 trans configuration, no "ring" was used on the OEM input shaft.

[Bill Clupper]

Re: Thank you Joe,...

You might also note that in the early case M-20 trans configuration, no "ring" was used on the OEM input shaft.