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OK, I have a 1971 LT-1, with a build date of Jan 1971 per NCRS, and which matches the VIN Sequencing (#6282 car built that year). However, I have a stamp code on the Muncie of P1T29A, which would be Dec 29, 1971 for a M20. However, the transmission is also stamped wn the proper VIN stamp matching my VIN number. Looking at the markings, it's looks original (quite warn). Am I to assume this is a replacement transmission, or at least casing, that someone stamped with a "proper" VIN stamp. Also thought most LT-1's came with the M21.
An M20 transmission was the standard trans you had to option for M21 or M22 . So the rear axle ratio would be one of the following 3:36 3:55 or 3:70 for an 1971 LT-1. For 1971 9224 Corvettes came with an M20 out of 21801 the other break down is M21 2387, M22 130 and M40 10,060. So yes your LT-1 could have an M20 and from your photo it appears to be a correct original stamps
OK, I have a 1971 LT-1, with a build date of Jan 1971 per NCRS, and which matches the VIN Sequencing (#6282 car built that year). However, I have a stamp code on the Muncie of P1T29A, which would be Dec 29, 1971 for a M20.
The stamp code is based on the MODEL YEAR and not the Calendar year.
You Stamp shows an assembly date of Dec 29th of the 1971 model year which would be Dec 29th 1970.
The stamp code is based on the MODEL YEAR and not the Calendar year.
You Stamp shows an assembly date of Dec 29th of the 1971 model year which would be Dec 29th 1970.
The first letter is P for Muncie H.D. 4-speed aluminum case. (This is heavy duty in relation to an aluminum passenger car transmission and is not to be mistaken for the M22.) The next number is the last digit of the model (not calendar) year (0, 1 or 2). A letter designates the month produced (see chart). Two digits are the day of the month. A suffix letter ends the code
(P) Muncie HD 4 Speed
(1) 1971 Model Year
(T) Month- Dec
(29) Day 29th
(A) Suffix Wide Range
Muncie 4-speed Passenger (Aluminum Case)
Suffix First Gear Ratio
A 2.52.1 - wide range M20
B 2.20.1 _ close range M21
C 2.20.1 - Rock Crusher M22
Calendar months produced are designated as follows:
A Jan
B Feb
C Mar
D Apr
E May
H Jun
K Jul
M Aug
P Sept
R Oct
S Nov
T Dec
Thanks PJO. What had me turned around was the December build date on the tranny. Didn't realize it was December of the prior year. That makes sense now.
Where is the code stamped on the housing? When I purchased my car I was told it had done m-21, but I'd like to verify myself.
Passenger side, on a boss at the end of the main case, just in front of where it joins the tail shaft housing. That info, together with the picture above, should be all you need.
Muncie didn't start to add the ratio identifier until 69 I believe, so if your car is 69 or later you will be able to ID the M-21 with a "B" stamp at the end of the date code.
You are correct 1968's did not have the letter stamped in the case. But here is a simple way to see what the transmission is if it is a known original
While casting numbers are a definite guideline for identification, a quick, somewhat accurate way to quickly identify Muncie transmissions involves the input shafts. The Muncie utilized seven different input shafts. All transmissions with a 26-spline input shaft used a 32-spline output shaft, and all those with a 10-spline input shaft was paired with a 27-spline output shaft.
Almost all Muncie transmissions were delivered with a groove(s) on the input shaft and counter gear. These grooves were cut into the input shaft so assembly line workers at the plant could easily identify which input shaft should be installed in a particular unit.
Input Shaft Identification
Year Type Grooves
1963-1965 M20 None
1966-1970 M20 Two
1970-1974 M20 Two
1963-1970 M21 One
1970-1974 M21 One
1967-1970 M22 None
1969-1974 M22 None
We said almost all Muncie transmissions had a groove(s) around the input shaft, and that is because most close-ratio Muncies will have one ring around the input shaft. But, those built between 1963 and 1965 will not have any rings around the input shaft. Starting with the 1966 models, the wide ratio versions got two rings around the input shaft, and the M22 input shaft had no rings and looks like a 1963 through 1965 wide-ratio input shaft. Also, there were some input shafts produced by the aftermarket and General Motors that have no identifying rings on them. This is a large reason why some guys will not rely on transmission identification by the input shaft grooves.
My old 69 Z28 with a October 18th M20 had no suffix after the build date.
I believe October 21st of 1968 is the date the A B C were using after the build date -
Muncie identification
