Asked in Chevy Corvette
Chevy Corvette

How do you tell if you have an m20 or m22 transmission in a 1974 Corvette?

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01/10/2007

How do you tell the difference between the (M20,M21) and the M22 transmission, you can not really tell by looking at the case ,the difference is the actual cut of the gears,the M20 and M21 have a much greater angle to the teeth of the gear and also will have a twist in the teeth (helical bend), the M22 gear is a more straighter cut gear teeth and also thicker and wider gapped teeth. 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

User Avatar
Wiki User
01/10/2007

How do you tell the difference between the (M20,M21) and the M22 transmission, you can not really tell by looking at the case ,the difference is the actual cut of the gears,the M20 and M21 have a much greater angle to the teeth of the gear and also will have a twist in the teeth (helical bend), the M22 gear is a more straighter cut gear teeth and also thicker and wider gapped teeth. 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 1970-74 years, however an early version of the M-22 was available as early as 1965 in Corvettes. Proper identification is 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 drivetrain 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 ( Year one )