A: 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.
4-speed manual transmissions ratios for both Muncie and Saginaw, with other Muncie data, are as follows:
Maincase Gear Ratios Input Grooves Cluster Output
RPO Years Casting 1st 2nd 3rd 4th Spline (Input) Pin Spline
-----------------------------------------------------------------------
Muncie
M20 63-65 3851325 2.56 1.91 1.48 1.00 10 None 7/8-inch 27
M20 66-67 3885010 2.52 1.88 1.46 1.00 10 2 1-inch 27
M20 68-69 3925660 2.52 1.88 1.46 1.00 10 2 1-inch 27
M20 70 3925661 2.52 1.88 1.46 1.00 10 2 1-inch 27
M20 71-74 3925661 2.52 1.88 1.46 1.00 26 2 1-inch 32
M21 63-65 3851325 2.20 1.64 1.28 1.00 10 1 7/8-inch 27
M21 66-67 3885010 2.20 1.64 1.28 1.00 10 1 1-inch 27
M21 68-69 3925660 2.20 1.64 1.28 1.00 10 1 1-inch 27
M21 70 3925661 2.20 1.64 1.28 1.00 10 1 1-inch 27
M21 71-74 3925661 2.20 1.64 1.28 1.00 26 1 1-inch 32
M22 65 removed 2.20 1.64 1.28 1.00 10 None 1-inch 27
M22 66-69 3925660 2.20 1.64 1.28 1.00 10 None 1-inch 27
M22 70* 3925661 2.20 1.64 1.28 1.00 10 None 1-inch 27
M22 71-74 3925661 2.20 1.64 1.28 1.00 26 None 1-inch 32
*Colvin reports that 454 Chevelle M22 in 1970 used the 1971-74 splines
M20 6-cyl 3.11 2.20 1.47 1.00
M20 V8 2.54 1.80 1.44 1.00
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, "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 tailhousing 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.
Muncie 4-Speed Transmission Gear Angles
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