Magical Muncie Four-Speed
Transmissions are an essential part of hot rod builds. One of the most common swaps involves a Muncie 4-speed. These are easy to adapt to a wide range of hot rods. There are three basic types of Muncies: M20, M21 and M22.
A Muncie differs from a Saginaw transmission in that its reverse lever is in the tail housing, not the side cover. The difference between a Muncie and a Borg-Warner is the Muncie’s 7-bolt side cover (two less than Borg-Warner units).
Many Muncie main cases, tail housings and side covers have casting date codes. The code consists of two ½-in. circles divided in half. One is the date marker and the other the status marker. One side of the date marker has a month designator 1-12. The opposite side has up to five dots that indicate the week of the month. The status marker was used at the Muncie, IN plant to keep track of problems and is generally not important to enthusiasts. Hot rod builders are usually not too concerned with codes, but some units are pretty rare.
A serial number is stamped on all Muncie transmission cases. The code has nine symbols that will reveal the GM division, year, model, assembly plant and car the transmission was used in. The first symbol indicates division, the second matches the last digit of he model year and the third tells what assembly plant the car was built in. The last six digits are the last six digits of the car’s VIN.
David W. West of Davids 4 Speeds LLC is a specialist who rebuilds “Big 3” four-speed transmissions and who can help you identify what you’ve got. About 95 percent of West’s work involves Muncies. West says all Muncies work on the same basic principle: The input shaft goes to the cluster gear. The cluster is mated to the speed gears. The speed gears are independent of the main shaft.
The magic of the Muncie is in the synchro assemblies. The synchro hub is splined on the main shaft. The slider and the synchro ring grabbing on the cone of the gear allows the slider to engage the engagement teeth on the gears.
West tells people to think of a transmission in terms of levers. The lever is engine power. It creates leverage just like extending the length of a wrench. If you take a one-foot wrench and increase its length by two feet, it will break a tight bolt loose. The principle of a transmission is the same. With the leverage and the gears combined with a lever action, you increase the power an engine makes.
The Muncie M20 is basically a wide-ratio transmission and was intended for normal street driving use. The early ones had a 2.56:1 first gear. The later 1966-1974 units had a 2.52:1 first gear. They are compatible with a GM 3 Series differential. They are for economy and street drivability and just cruising.
The Muncie M21 is a close-ratio unit originally designed to be matched with 4.11:1 or 4.56:1 GM 4 Series differentials. Of course, rodders get a little creative. The M21 was for higher performance cars, which makes it a great box to use in a rod. A close-ratio gearbox keeps shift points closer together, which keeps rpms up. With a close-ratio box a driver can run through the gears and keep the engine in its power band so it performs in street racing or drag racing.
The M22 has higher-nickel-content gears of straight-cut design. It uses much the same gear ratios as the close-ratio M-21, but it can handle much more abuse. An M22 retains less heat due to the straight-cut gear design and the lack of thrust that helical-cut gears create. An M-22 holds up well. The downside is that it tends to make noise. It will rattle at low rpms and whine at higher rpms. The M22 is called the Rock Crusher. “Dump the Rock Crusher,” hot rodders say. People think all Muncie four-speeds are Rock Crushers, but they’re not.
Nowadays there are custom gear ratios and set ups, including M21 wide-ratio and M22 wide-ratio units. You can pretty much get what you want. Beefed up boxes for hot rods and Resto-Mods are popular, too. “
When rebuilding a Muncie four-speed you can take a ’65 case and put in later year parts. West bores the cluster pin and puts in a 1-nch pin. Then, he can put in a later synchro assembly and have the modern functionality with old looks.
Synthetic oils shouldn’t be used in a Muncie. Synthetics are not “synchro friendly.” They are so slippery they don’t allow synchro rings to grab on to the cone of the gear and synchronize properly. This will lead to grinding the gears. Synthetics also have a different consistency then a natural fluid. The early Muncie cases, especially, tend to be more porous so they don’t hold up well with a synthetic. The synthetics leak out of them more easily than natural gear lubes.
GL5 fluid is another choice these days. It is said to be superceding GL4. GL5 has not been around as long enough to know how it’s going to unfold. West thinks GL5 has sulfur in it that’s corrosive to brass. Like a synthetic, it doesn’t allow synchro rings to grab the cones of the gears. So, stick to a GL4 for now
One of the main transmission issues is shifter problems. Grit that gets into the grease in the shifter mechanism can cause real problems. People also fail realize how important the shifter geometry is. A shifter handle has a specific bend to it.. Anything that impedes the shifter’s movement can keep a shifter from going in and going as far as it has to.
Muncie shifters have three selector plates coming out the bottom that have a unique bend and unique length that corresponds with the arm that’s attached to the shift shaft. The rods are unique to each application. If you mix and match randomly, the length and shape of the bends will change and affect the shifting geometry. Having the proper geometry is a necessity. Often a hot rodder will say his transmission is grinding in fourth, but geometry is the problem.
Davids 4 Speeds LLC www.Davids4Speeds.com
Diagram shows the parts and basic workings of a GM Muncie 4-speed.
This worn out Muncie synchro hub is of a type used from 1966-1974.
The 27-spline tail shaft housing.
Some Muncie parts are very valuable to Corvette collectors, so check each one’s rarity before putting one on your hot rod.
Shifter handle design and the geometry of the linkages is very important.
Cut-away display shows the public what’s inside a Muncie 4-speed.