The primary Muncie 4-speed transmission was marketed under Regular Production Option (RPO) M20. M20 was the generic
RPO for a 4-speed, so note that the Saginaw 4-speed was also marketed under RPO M20, but with slightly different gear ratios in a
significantly different case. In addition to the M20, a close-ratio 4-speed was sold as M21, and a Heavy-Duty close-ratio 4-speed was
sold (at a much higher price and much more limited application) as M22.
The buyer had no choice as to the transmission manufacturer; use of the aluminum-case Muncie or the cast-iron-case Saginaw was determined
for you by the factory as a function of your model. M21 and M22 transmissions were even more limited to specific models, with details of
the limitation depending upon the model year.
To distinguish between Muncie types, if the Muncie was built after 21-Oct-1968, the plant assembly-date stamp contains one of three
letters at the end of the build code:
A = 2.52:1 Wide Ratio (M20)
B = 2.20:1 Close Ratio (M21)
or
2.42:1 HD Close Ratio (MC1 3-speed)
C = 2.20:1 HD Close Ratio (M22 4-speed) aka, "The Rockcrusher"
This suffix was used on all Muncie assembly plant stamps from 21-Oct-1968 through 1974. See the transmission decoding FAQ for other
details. The assembly plant stamp is normally located on the passenger side of the transmission, arranged vertically just in front of the
tail housing joint.
If the input shaft is visible, the number of circumferential grooves on the input shaft will also generally indicate the Muncie type. Be
careful, as 1963-65 M20 Muncies are like later M22s in that they have no input shaft grooves; however, the early M20 has only a 7/8-inch
diameter cluster pin, while the M22 has a 1-inch pin.
To distinguish an early (pre-suffix) M22 from the other Muncies when the transmission is on a car, note that all M22s had the lower,
forward, passenger-side maincase boss drilled and tapped for a magnetic drain plug. This wasn't done (by the factory) for M20/M21 until
1970. While this boss could be drilled for a plug on a M20 after it left the factory - if the boss is undrilled then the case definitely
was not part of a M22.
The M22 gears have a shallower mesh angle to reduce thrust load and heat. If the gearbox cover is removed, the difference in the angle
between the M22 gears and the gear angle used in the other Muncies is discernable. Note in the photo below that on the M20/M21 the bottom
of the next tooth starts below the top of the preceding tooth (i.e., they overlap), whereas on the M22 the angle is such that the teeth
actually have a small gap between the top of one tooth and the bottom of the next. |
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