The primary four-speed manual transmissions used by Chevrolet are the Saginaw, Muncie and Borg Warner models, with Saginaw and Muncie units being specifically made for Chevrolet. Saginaw and Borg Warner models are normally used on general purpose applications. Muncie transmissions are known for both high-performance and heavy-duty applications, with three different variations available: the M20, M21 and M22. Identification begins by visually identifying the units as Saginaw, Muncie or Borg Warner models and then by differentiating the three Muncie units visually and by decoding the serial number.
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Inspect the transmission. Both Muncie and Saginaw transmissions have 7-bolt side covers, while the Borg Warner has a nine-bolt side cover, according to Year One.com. Also, the reverse lever on the Muncie units is mounted in the extension housing, while the Saginaw reverse lever is mounted in the side cover
Locate the serial number to differentiate 1969 and up Muncie transmissions. The transmission case has various codes including the casting code and the serial number. The serial number provides the date of manufacture, however, on 1969 and later transmissions there is a letter code at the end which identifies the gear-ratio, which is different on all three Muncie models. An example of the serial number is "P4D23B."
Decode the serial number. According to My SS.com, the serial number example "P4D23B" decodes as an M21 Muncie transmission manufactured on April 23, 1974. The "P" in the first position designates Muncie, "4" designates the year, 1974, and "D" designates the month, April, with "A" representing January and "T" for December. The Letters F, G, I, L, N,O and Q are omitted. The next two digits are the day of the month, the 23rd, while the "B" in the last positions designates the Muncie M21, with "A" for the M20 and "C" for the M22.
Identify Muncie 1963 to 1967 transmissions. These Muncies have the same serial number format, minus the gear ratio designation, so a spline and gear tooth count must be done. According to Nasty Z28.com, 1963 to 1965 M 20 transmissions have 10 splines and 24 input gear teeth, while 1966 and 1967 models have 10 splines and 21 input gear teeth. Both the M 21 and M 22 have 10 splines and 26 input gear teeth, however, the M 22 has "straight cut" gears, which are not as pointed as M 21 gears.
Tips and warnings
- The input shaft runs from the transmission to the engine and is part of the transmission. The splines are on the shaft going into the engine and the input gear goes into the transmission. This is illustrated at the top of the Year One Tech page, while the input gears are illustrated at the bottom (see Reference 2).
- Since the Muncie M 22 is a heavy-duty transmission, it is usually paired with high-torque big block engines and trucks.
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