Muncie M20/M21/M22 Main Case Identification Chart
If you're working on a classic car or restoring a muscle car, chances are you'll encounter a Muncie 4-speed transmission. These transmissions were produced from the 1960s through the mid-1970s and were widely used in GM vehicles. The Muncie 4-speed is highly regarded for its durability and performance, but identifying the correct main case can be a challenge. In this article, we'll provide a Muncie M20/M21/M22 main case identification chart to help you determine which transmission you have.
What is a Muncie Main Case?
The main case is the housing that contains all the gears and other internal components of the Muncie transmission. The main case is usually made of cast iron and has several casting numbers and codes that can help identify the transmission. The casting numbers and codes are located on the main case itself and can be found by looking on the passenger side of the transmission.
Muncie M20 Main Case Identification
The Muncie M20 is the most common of the Muncie 4-speed transmissions. It was used in many GM vehicles from 1963 to 1974. The Muncie M20 main case has a casting number of 3831704 and a casting date code of "P" (for example, P8R01). The M20 has a 2.52:1 first gear ratio and is often referred to as the "wide ratio" Muncie.
Muncie M21 Main Case Identification
The Muncie M21 is the close ratio version of the Muncie 4-speed. It was used in many GM vehicles from 1963 to 1974. The Muncie M21 main case has a casting number of 3851325 and a casting date code of "P" (for example, P8R01). The M21 has a 2.20:1 first gear ratio and is often referred to as the "close ratio" Muncie.
Muncie M22 Main Case Identification
The Muncie M22 is the rarest and most desirable of the Muncie 4-speed transmissions. It was used in only a few GM vehicles from 1965 to 1974. The Muncie M22 main case has a casting number of 3925660 and a casting date code of "P" (for example, P8R01). The M22 has a 2.20:1 first gear ratio and is often referred to as the "rock crusher" Muncie due to its aggressive shifting.
Other Identification Factors
In addition to the casting numbers and codes, there are several other identification factors to consider when trying to identify a Muncie main case. The input shaft has 10 splines on the M20 and M21 and 26 splines on the M22. The output shaft has 27 splines on all three transmissions. The Muncie also has a distinctive side cover with seven bolts and a reverse shift lever located on the extension housing.
Conclusion
Identifying the correct main case for a Muncie 4-speed transmission can be a challenge, but with the help of our Muncie M20/M21/M22 main case identification chart, you should be able to determine which transmission you have. Remember to look for the casting numbers and codes on the passenger side of the main case, and consider the other identification factors such as the input and output shafts and the side cover. With the right information and a little bit of patience, you'll be able to identify your Muncie transmission and get back to enjoying your classic car or muscle car.