Muncie 4-Speed Transmissions. Paul Cangialosi

       These are three first-speed Muncie gears. The far left gear is a 1963 type that has the recess for the 1-2 synchronizer snap ring and a smaller bore. The middle gear is the later-style original-equipment late-1964 to 1974 gear. On the right is an aftermarket gear made in Taiwan.

      • M21 first, 2.20:1; second, 1.64:1; third, 1.28:1; fourth, 1.0:1; and reverse, 2.27:1

       1964–1965

      Two major improvements were issued. The first was the introduction of a larger-diameter front bearing that meant a new case casting and larger-diameter front bearing retainer were necessary. The second was that the first-speed gear now rode on a bushing that was press-fit onto the mainshaft. It stopped against the first and second synchronizer assembly, thereby eliminating the need for the assembly to have a retaining snap ring. Because the bushing was subsequently retained by the rear bearing, the synchronizer could not go anywhere. The first-gear thrust washer was eliminated and the gear was designed to have a thrust surface that ran against the rear bearing’s inner race.

      The first-gear design change was done for several reasons. The first was added strength. Whenever you have a snap-ring groove between a flow of power you have a potential stress riser on the shaft. Because the slider engages first gear across the snap-ring groove, a huge stress riser develops that leads to broken mainshafts. First gear also had a tendency to seize to the mainshaft. Cutting grooves and valleys for oil in the shaft only weakened the shaft more. A bushing was used with a “v” notch to promote better oil flow under the gear.

      By the end of 1965 the rear extension housing saw some modifications to the casting. Webbing was added to the top and bottom. Small changes in countergear needle bearing spacer tubes surfaced. Some tubes were seamless with four needle spacers while others had a seam with six spacers. There seems to be no specific time when this change took place. By 1965 the shifter-shaft designs changed because they had been snapping. The new thread size of the stud was increased to 3/8-24. In 1964 and 1965, Muncies in some of the full-size Chevrolet Impalas and Pontiac Catalinas were equipped with longer mainshafts and extension housings to keep driveshafts shorter and reduce harmonic vibration.

       Muncie shifter shafts have evolved in three basic stages. From the left, the small 5/16-18 threaded stud, which snapped easily. The newer 3/8-24 stud still had to fit the rectangular keyway of the linkage arm, and so it had flats milled on each side, but they still broke. The last revision was a bolt-on shaft using a standard 3/8-16 threaded hex head bolt.

       These spacer tubes go inside the countergear. The upper tube has no seam but the lower one does. Because the needle bearings ride against the seam, extra spacer rings are needed to cover the seam. Typically, the seamless spacer had four needle-spacer rings. The seamed spacer tube used six. Most of the later GM overhaul shop manuals show four spacers in the exploded-view diagrams when, in fact, the transmissions used six.

       M22 RockCrusher

      In 1963 the Corvette Grand Sport racing program was instituted. The early Grand Sports used a special heavy-duty version of the M21 close-ratio transmission. These special units evolved into what is called the M22 today. According to research by Alan Colvin (author of the Chevrolet by the Numbers books), 57 M22 units were actually built for 1965 production. The engineering change documentation for the M22 is dated December 12, 1964.

      The change basically states that a new gearset is to be used with different synchronizer assemblies, the main case is to be modified to accept a drain plug, and the countershaft bore of the case is to be machined to accept a 1-inch-diameter countershaft. A letter to Zora Arkus-Duntov dated December 8, 1964, is referenced in this engineering change stating successful use of the M22 in Grand Sport Corvette field testing.

      So exactly what is an M22? The RPO M22 stands for Heavy-Duty Close-Ratio. Many people think the gearset had some different alloy compared to the standard sets, but it didn’t. According to original engineering drawings I have of the M22 first gear, it is made of an 8620-alloy steel. The same alloy is used to manufacture the M20 and M21 gears. The difference is the notation on the drawing to add shot peening to the gears.

      Shot peening is a process in which the gear is blasted (like sand blasting) with steel shot. Steel shot is spherical and the gear surface develops thousands of microscopic dimples when the shot hits the gear. These dimples reduce stress risers on the area’s gear teeth that can develop cracks because of fatigue.

       This is a pair of Muncie first gears. The gear on the left is the standard M20 and M21 and the gear on the right is the M22. I placed them back-to-back so you can see the difference in the helix angles of the teeth. The M22 is straighter.

      The tooth counts and gear pressure angles of M20, M21, and M22 gears are the same. The difference is the helix angle. If you reduce the helix angle of the gear you reduce thrust loading on the main case. Reduced thrust loading reduces heat and yields less horsepower loss to the rear wheels, but it increases gear noise. Hence the name “RockCrusher.” Muncie 4-speeds have varying helix angles in the gearsets. Typically, the M20/M21 gearsets have a first-gear helix angle of 26.4 degrees and an input shaft angle of 39 degrees. The reduction in the angle with the M22 is quite substantial. The M22 first gear has an angle of 14.5 degrees and the input shaft is 24.2 degrees.

       The standard first-gear sleeve is on the top and the sleeve for the M22 is on the bottom (GM PN 3932228). It has flats ground into it to promote better oiling so that first gear does not seize to it. In road-racing applications, when you are in fourth gear doing more than 100 mph, first gear is spinning on the mainshaft at more that twice the mainshaft’s RPM. This is one of those undocumented parts that is not listed in all the parts books but takes some digging to find. Later you could get a roller bearing M22 first-gear assembly directly from Chevrolet (GM PN 3965752). The roller first gear was designed to prevent gear seizures in high-speed road-race conditions.

       1966–1967

      Several major improvements began in 1966. The diameter of the main case countershaft bore was officially increased to 1 inch. The most common main case casting was 3885010. This larger diameter was necessary because the big-block and small-block engines were producing more power. As a result, all M20, M21, and M22 countergears had to be redesigned to accommodate a larger countershaft. The needle bearings changed from .156 to .125 inch and the diameter of the spacer tubes also changed. New thrust washers for the countergear were also needed because their bore size changed and the location of the holding tang of the thrust washer was also redesigned.

       The early 24-tooth maindrive is on the left and the later 1966–1970 model with 21 teeth is on the right. Reducing the number of teeth made a huge difference in reducing breakage of this gear; it becomes stronger while keeping the same gear diameter. This gave it a thicker tooth profile. I always use an apple pie as an analogy. A pie divided into four equal pieces obviously has larger pieces than the same pie divided into eight pieces.

      The synchronizer assemblies were also updated to what is commonly called a “shoulder style” synchro ring. The early 1963–1965 ring had a tendency to crack at the strut key slot. Therefore, the ring was redesigned with material added to create a shoulder in front of the synchro teeth. It’s important to know that the later rings used a narrow synchronizer hub to compensate for this increased thickness. If you mix them up you end up