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How to Identify a Muncie Four Speed: Build Date Codes, VINs, and Input Shaft

Technically Speaking: Get It In The Right Gear, Part 2

James Berry Jul 29, 2016
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Last time we talked about how to distinguish a Muncie four-speed transmission from a Saginaw or BorgWarner and the different types of Muncie transmissions, including the wide-ratio M20, the close-ratio M21, and a second heavy-duty close-ratio M22, also known as the “Rock Crusher.”

We also discussed the best ways to identify a Muncie, which is by the casting numbers on the external components of the transmission. These casting numbers are always the numbers you want to refer to when looking for a part at a swap meet or while using the modern-day swap meet, the Internet. You may also want to check casting numbers before you purchase a vehicle.

Production Code Number (Build Date Code)

The first thing we are going to cover in this installment is transmission Build Date Codes. This code will be stamped onto the main case. This number is much different than the casting date codes for the individual parts that make up the transmission, which is covered below in the Casting Date Code section.

Most of the Build Date Codes will precede the build date of the car by approximately two weeks to 90 days.

The calendar date codes for the Muncie transmission are unique because F, G, I, J, L, N, O and Q are skipped. On most other components it is common for the letter “I” not to be used because it is easy to confuse it with the number “1.”

Muncie Transmission Calendar Date Codes
A=January E=May P=September
B=February H=June R=October
C=March K=June S=November
D=April M=August T=December

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Build Date Code Location

1. 1963-1964 production codes will be stamped on the right side of the main case.
2. 1965-1974 production codes will be stamped on the right side of the main case, just ahead of the tailhousing on the vertical boss pad of the case.

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Build Date Code Breakdown

1963-1966 production codes will start with the letter P and have four numbers, for example: P0421.

1. P represents the manufacturing plant, Muncie.
2. 04 represents the month code when the unit was assembled, April.
3. 21 represents the day of the month the unit was assembled, the 21st day of April. 1967-1974 production codes will start with the letter P and have five numbers, for example: P9D09.

1. P represents the manufacturing plant, Muncie.
2. 9 represents the model year.
3. D represents the month code when the unit was assembled, April.
4. 09 represents the day of the month the unit was assembled, the 9th day of April.

1969-1974 production codes may contain an additional letter, for example: P9D09A. This was done to aid in identifying the transmission ratios. Some early 1969 transmissions did not have this additional letter.

Casting Date Code

It is importing to understand that most all Chevrolet parts have some kind of date coding cast or stamped into the part.

Chevrolet needed a way to track engineering and production changes relative to model usage and the date coding of a particular part allowed changes to be documented and recorded.

There are two types of date coding. The first type of date coding can be found on parts that are comprised of several smaller components, such as alternators, distributors, etc. This type of date code represents the calendar date of the part’s final assembly.

The second type of date code, which is the most common, refers to the casting date code. This type of date coding is used on engine blocks, heads, transmission main case, transmission tailhousing, transmission side cover, etc. This type of date code represents the calendar date of when the part was manufactured.

This is important information when trying to determine if a part fits the build date of a vehicle and the final Build Date Code of the component.

Most of the Muncie four-speed transmissions will have a casting date code on the main case, extension housings, and side cover.

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1. The main case date code is usually found on the passenger side of the case just below the casting number.
2. The extension housing date code can usually be found on the passenger-side of the case within a few inches of the casting number.
3. The side cover date code is usually found near the casting number.

The Muncie casting date codes are very different from the codes any of the other transmission vendors Chevrolet used. These codes can be difficult to read on some units and are comprised of two circles that are divided into two halves.

One circle is the Date Marker and the other circle is referred to as the Status Marker.

One half of the Date Marker circle will have a month designator for when the part was cast. The month is determined by numbers 1 through 12, which correlate with the months of the year. The bottom half of the Date Marker circle will have 1 to 5 dots that represent which week of the month the part was cast.

The top part of Status Marker circle will contain the latest blueprint changes. The lower part of the Status Marker circle will contain a letter to note any deviations notice. It can be acceptable for the Status Marker circle to be blank. The Status Marker information was used by the Muncie plant to help track a problem part.

Remember, all of these casting codes should precede the final production code of the transmission. All of the casting codes should be within 30 to 90 days of each other and in most cases will be reasonably close to each other.

Vehicle Identification Number Stamped Into The Main Case

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Another way to confirm that the transmission is original to your car is most Muncie four-speed transmissions will have a Vehicle Identification Number (VIN) number stamped on the main case.

The engine and transmission were stamped at the same time by the final assembly plant. The stamping method used could be “gang” or ‘individual” stamped, this method varied and was chosen by the assembly plant.

The location of the VIN stamp can vary but it can usually be found on the top leading edge of the main case or on a mounting surface near the Build Date Code.

The number of characters in the VIN stamp varied from plant to plant. However, the Corvette assembly plant in St. Louis from 1962-1964 was relatively consistent. The first character was the last digit of the model year. The following six digits were the last six digits of the VIN.

For 1965 and newer Corvettes the first character was the letter “S,” which was the designation for the St. Louis assembly plant. The following six digits were the last six digits of the VIN.

Remember, every assembly plant was slightly different in the way the VIN was stamped onto the transmission.

Input Shaft Identification

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There is another way to quickly identify Muncie transmissions. All Muncie transmissions have a ring groove on the input shaft and counter gear. These grooves were cut into the input shaft so the Muncie assembly line worker could easily identify which input shaft should be installed in a particular unit. This chart should help with Muncie identification.

Input Shaft Output Shaft
Year Type Rings Splines Tooth Count Splines
1963-1965 M20 None 10 24 27
1966-1970 M20 Two 10 21 27
1970-1974 M20 Two 26 21 32
1963-1970 M21 One 10 26 27
1970-1974 M21 One 26 26 32
1967-1970 M22 None 10 26 27
1969-1974 M22 None 26 26 32

Well, there you have it. Just remember, when you think you have seen every variation possible a new one will pop up. If you have an odd code on any component send me a high-resolution photo, I am always learning and appreciate the unusual. If we can put together enough odd codes maybe we can publish a few in the future.



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