How to tell the difference between M20-21-22

[Michael F.]

Re: How to tell the difference between M20-21-22

Here's and example of the assembly and suffix code that John mentions, located on the passenger side, on the rear stamp pad at the junction of the maincase and the tail housing. This one shows P1D19C, which translates Muncie IN, 1971 model year, April 19th, C = M22 [A= M20; B = M21].

The 5-digit number, 43265, above and to the right is the numbering system for a GM over-the-counter Muncie transmission. Believe it started at 35000 each year. To the left of that # (not shown) is stamped "CT1" which is Chevrolet Transmission, 1971.

I have a Muncie M-20 with the code P0910. Is that a passenger case?

[Bruce T.]

Re: How to tell the difference between M20-21-22

P0910 is the assembly date code. That format was used on Muncies 1966 and earlier. 0910 indicates the 9th month and 10th day; Sept. 10. The format changed starting with the 1967 model year to include the model year. Example: P7T10 is (T) December 10, 1966 since the model year is 7 (1967).

[Wayne M.]

Re: How to tell the difference between M20-21-22

I have a Muncie M-20 with the code P0910. Is that a passenger case?

Could be -- depends on the VIN stamping and location. Here's my '...325' original '65 trans date stamp on the passenger side. This location and the grind indicate prior to mid-September 1964. After that, they dropped the grind step. Second pic is of the driver side with the VIN (both the earlier format and the S-only format for the rest of the 1965 model year. If your assembly date P0910 is on the lower rear corner of the trans driver side cover, then it's '63 or '64 (Sept 10th). If the VIN is stamped on the top edge (front or back) of the maincase, then probably a passenger unit. Or, if the VIN is stamped in proper location, but the number doesn't correspond with the birthday of assembly, then again, passenger. The Pmmdd (month/day) format was used also on the 1966 model year '...010' cases.






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[Michael F.]

Re: How to tell the difference between M20-21-22

P0910 is the assembly date code. That format was used on Muncies 1966 and earlier. 0910 indicates the 9th month and 10th day; Sept. 10. The format changed starting with the 1967 model year to include the model year. Example: P7T10 is (T) December 10, 1966 since the model year is 7 (1967).

Mine is a 63 M-20 2.54 wide ratio. I think it and the bellhousing are original to the car.

So there are no yr indicators before '67 ?

[Wayne M.]

Re: How to tell the difference between M20-21-22

.....So there are no yr indicators before '67 ?

Correct -- at least none found in the trans assembly code stamped at Muncie IN. Only reference to model year is in the VIN added at final assembly plant.