GM Muncie 4-Speed Decoding

The Muncie 4-speed transmission was used with many different GM models and engine combinations from 1963 through 1974. The basic design and operation theory of the gearbox changed little during this period, however several design upgrades and modifications were made during the total production run. The Muncie was used with most performance engines, and is not to be confused with the Saginaw or Borg Warner 4-speeds.

The Muncie is distinguished from the Saginaw in that the reverse lever on the Muncie is mounted in the extension housing, where the Saginaw reverse lever is mounted in the side cover. The main difference between the Muncie and the Borg-Warner is that the Muncie has a 7-bolt side cover and the Borg Warner has a 9-bolt side cover.




Muncie 4-speeds were produced in two different ratios wide ratio (M-20) and close-ratio (M-21). An extra heavy duty close-ratio version (M-22) was also offered on many of the big-block high-performance models.The choice of transmission was dictated by the engine size and rear-axle ratio. Axle ratios of 3.73 and lower (numerically higher) came with close-ratio transmissions, while axle ratios of 3.55 and higher (numerically lower) used wide-ratio transmissions. In addition, many GM high-performance engines came with M-22 “Rock-Crusher” in the 1970-73 years, however an early version of the M-22 was available as early as 1965 in Corvettes.

Proper identification of the transmission type is absolutely necessary, but sometimes difficult due to several factors. First, GM used several different methods to label and identify transmissions from year to year and model to model. Unfortunately the identification systems seem to have many exceptions, rendering them virtually useless to the restorer. In addition, due to engine and transmission swaps, and modifications that occur over the years many cars do not carry the original drive train that they were produced with.

For example, due to interchangeability, a 1969 Pontiac could have a trans case and gears from a Buick and a rear housing (extension) from an Oldsmobile. Several production changes through the years that effect the external appearance may also cause additional confusion when identifying a particular transmission:

1. Both single and dual drain plugs were used.
2. Speedometer adapters were used on either sides of the rear section (extension).
3. Side covers were sometimes secured with studs and nuts, other times with bolts.
4. Input shafts may be fine or coarse spline. Output shafts may be large or small. Combinations of these vary.
5. External shift levers are secured with studs and nuts, others with bolts. Some levers are long and some short.
6. Input shafts may have 1 ring, 2 rings, or no rings at all.

M-22 “Rock Crusher” Identification
The M-22 close-ratio transmission is easily identified internally from the M-20 and M-21 versions by the angle of the gear teeth.

The M-22 was only produced in a close-ratio version, so it will not appear on models with high rear-axle ratios (3.55 or lower numerically) unless someone has changed it.

The main internal differences in the 3 types of transmissions are listed in the parts below.
• M-20 differs from M-21: Input shaft and Cluster gear
• M-20 & M-21 differ from M-22: Input shaft, Cluster gear, First, Second, Third, Reverse idler gear

All other components interchange, with the exception of production upgrades and changes. For example, 1963-65 synchronizers, blocking rings, etc. do not interchange with 1966 and newer versions, and therefore the transmission must be stock or interchanged as a unit.


M-20 Wide Ratio
Cluster gear teeth: 25-22-19-17
Input shaft/gear teeth: 21

M-21 & M-22 Close-Ratio Cluster Gear teeth:
27-22-19-17
Input shaft/gear teeth: 26