Gear Ratios
M20
First, 2.52:1
Second, 1.88:1
Third, 1.46:1
Fourth, 1.0:1
Reverse, 2.59:1
M21
First, 2.20:1
Second, 1.64:1
Third, 1.28:1
Fourth, 1.0:1
Reverse, 2.27:1
M22
First, 2.20:1
Second, 1.64:1
Third, 1.28:1
Fourth, 1.0:1
Reverse, 2.27:1
(heavy duty)
Prior to 1966 order options really didn’t exist for Muncies. General Motors optioned cars with either the M20 or M21 ratios based on engine types and axle ratios. Many window and tank stickers exist from before 1966 that list cars as having M20s when in fact they had M21s. M22 production was rare in both 1966 and 1967; 29 were produced in 1966 and only 20 in L88 Corvettes in 1967.
Speedometer fitting placement was also changed. Before 1966 the speedometer gear “bullet” fitting on the extension housing was located on the driver’s side in the middle of the shift linkage and below the oil level. The passenger-side speedometer extension housing also has added material between the upper shifter mounting holes. These extension housings always had a tendency to leak as well as get in the way of aftermarket shifters. For some reason, Pontiac kept the driver-side speedo while Chevrolet and Oldsmobile did not.
A small update was also added to the pivot pin on the sidecover that holds the whole internal interlock and detent system. It was press-fitted into the cover but it had a tendency to fall into the transmission. The new pin design simply added a hat to the end of the pin so it could not fall through.
Webbing has been added to late-style extensions (casting PN 3857584). The passenger-side speedo tailhousing is always desired because it gets the driver-side speedo away from the linkage. Looking for the webbing is an easy way to recognize the casting when looking for parts.
The left countershaft is 1 inch in diameter and the right is 7/8 inch. Sometimes it’s difficult to see the difference. A great way to restore and upgrade worn-out pre-1966 cases is to just bore them out to fit the later shafts and upgrade the gearset. The larger countershaft is needed to handle the load of big-block engines. The larger the shaft, the more surface area the case has to support it.
The 1963–1965 needle countergear needle bearing on the left is .156 inch in diameter. Four rows with 20 needles per row were used. The later design reduced the diameter of the needle to .125 inch because of the larger countershaft, which used four rows with 28 needles per row.
This is essentially an original 1964 442 Oldsmobile Muncie casing with the original factory shifter. The sidecover pivot pin has no “hat” and can fall into the transmission. The speedometer fitting is located right where the linkage is. It’s also below the oil level and prone to leaks. By 1966 it was relocated to the upper passenger side.
These are the two types of sidecover pivot pins that were used. Always use a later-style pin (right) when doing a rebuild. Some people simply pressed out the old pin and welded a blob of metal to the end of it.
Beginning in 1967, transmissions had a date designator added to the serial numbers. For prior years only a month and date were added.
1968, 1969 and 1970
The 1968 model was identical to the 1967 model except for a main case casting alloy change. The most common main case casting used during this period was 3925660. The front bearing retainer was also changed to a thicker casting. The height of the casting changed from roughly .325 inch to .450 inch. This made piloting the transmission into the bellhousing an easier operation.
If you are going to switch extension housings so that you can use a passenger-side speedometer fitting, you must make sure the mainshaft can accommodate the different position of the drivegear. The upper shaft has the single position for the driver’s side and the lower shaft has more area added to the back for the passenger’s side. The hole in the shaft was actually used on 1969–1970 models that had a clip-on plastic gear.
The late-style synchro hub and blocking rings (left) were used on 1966–1974 transmissions. Adding more material in front of the teeth reinforced the blocking rings. The hub had to be narrowed to make room for this additional material. The early hub and ring combination is on the right. Early hubs measured 1.150 inches across the spline face and later hubs measured 1.020 inches.
More M22s were produced from 1968 to 1970 than in any other period. Using data from Alan Colvin’s Chevrolet by the Numbers books there were approximately 13,700 M22s made in the era compared to approximately 6,400 made in 1971 and 1972. What that means is that from 1968 to 1970 there were more factory 10-spline-input M22s assembled than the later 26-splines.
Notice the thicker shoulder of the later-style ring (left). The early ring (right) had a problem: Cracks developed at the key slot. Mixing early rings with a late hub causes excessive clearance and ring damage.
The 1969 design changes were subtle, and now all models had drain plugs as a standard issue. It was no longer just an M22 thing.
This was also the first year of “bolt-on” shifter shafts. Shifter shafts damaged because of broken studs or stripped threads were now a problem of the past. The rectangular drive portion of the shifter shaft was also increased in length from .605 to .722 inch. Shifter linkage arms were changed because the drive slot now had to be longer to match the drive portion of the mating shaft. The slot width of .315 inch remained unchanged. Putting a later linkage arm on a pre-1969 shaft can cause the linkage to become loose and out of alignment.
The speedometer drivegear only came as a molded 8-tooth gear. It was held in place with a spring-steel clip. It was obviously a move to save money and consolidate inventory since all transmissions would be assembled internally using all the same parts.