American Chevy Muncie Transmission Parts – M20, M21, & M22

Muncie (General Motors) 4-speed longitudinal transmissions were developed in three distinct models – The Muncie M20 wide ratio manual transmission, the Muncie M21 close ratio manual transmission and the Muncie M22 heavy-duty close ratio manual transmission. Whichever of these three transmission types you're dealing with in your vehicle, you can expect to find a complete selection of Muncie M21 transmission parts, Muncie M22 parts and Muncie M20 parts, including Muncie M20 rebuild kits.

We have the best prices on Muncie 4 speed parts, including Chevy Muncie 4 speed transmission parts, and we can guarantee you a great selection of items extending from the lowest cost seal or bushing to a complete replacement wide ratio or close ratio M22 gear kit.

More Information on Muncie M20, M21 & M22 Transmissions

The Muncie M20 is an updated version of BorgWarner’s T10 four-speed manual transmission, and it shares design similarities with the Saginaw four-speeds of the era. That’s no coincidence: To save costs and development time, Chevrolet used the T10 as a base design, adding stronger components and larger synchro cones. This makes the M20 much stronger than the Saginaw or T10. The 4-speed Muncie came in three versions. Both the wide-ratio M20 and close-ratio M21 debuted in 1963 in the Corvette. The M22 “Rock Crusher” followed four years later. It uses stronger components to handle big-block power.

The original M20 was only offered for one year. It has the GM casting number “3831704” on the case. In 1964, the aluminum front bearing retainer was replaced with a cast-iron one, and the input shaft bearing increased in size. This version stayed in production until 1974. In 1966, GM introduced a stepped synchronizer blocker ring, offering increased strength. From 1971 onward, all transmissions used a 32-spline output shaft.

Contrary to popular belief, you cannot tell a 4-speed Muncie is an M22 by looking at the input shaft splines. All M22s use a 26-spline shaft made from a stronger alloy than the other 4-speeds. However, GM made both 10- and 26-spline versions of the M20 and M21. There are two ways you can tell the difference between the M21 and the Rock Crusher. The M22 has 27 teeth on the countershaft gear, while the M20 and M21 have 29 teeth. M20 transmissions have two grooves on the input shaft, M21 transmissions have one groove, and M22 transmissions have no grooves. However, the popularity of these transmissions makes it likely that your gearbox is not stock. Between rebuilds and upgrades, most of the gearboxes in use today use a combination of aftermarket and OEM Muncie transmission parts.

What makes the Rock Crusher so popular in street rods and drag cars? The M22 uses high-nickel-content-alloy straight-cut gears that have less of an angle than the M20 and M21 gears. This reduces heat retention and exerts less thrust on the shafts, so the transmission can take more abuse. This design creates the distinctive low-RPM rattle and high-RPM whine the Rock Crusher is known for. Gear ratios are the same as the close-ratio M21. The Muncie four-speed was used in a wide range of GM products through the 60s and 70s. However, the Rock Crusher was only equipped in cars that had high-performance big blocks, including the Corvette, Camaro, Firebird and Chevelle.

Thanks to the similarities between the M20, M21 and M22 transmissions, you can use a combination of Muncie 4-speed transmission parts to build the transmission you want for your project car. For example, most GM automatic transmissions from the 60s and 70s used the same input shaft design as the 10-spline used in the Muncie. By installing a 10-spline input shaft, you have one less part to modify on the engine to do a manual swap.

Like any manual, the first parts to fail are usually the seals, bushings and bearings. Of these parts, the front shaft bearing and rear seal are usually the first to go. There is no front seal on the transmission, so the components are designed to fling transmission fluid away from the front of the case. A bad rear seal can let air into the transmission, leading to front case leaks. Most synchro failure is caused by poor clutch adjustment. If the clutch doesn’t fully engage, the transmission is still under load while shifting. Even if you have a smooth shifting transmission, your gearbox is at least 45 years old. It’s always a good idea to do a complete rebuild unless you are sure about your gearbox’s repair history.