Transmission swaps are an essential part of building a hot rod. One of the most common swaps you see involves the General Motors Muncie four-speed gearboxes. They are very popular transmissions with “Bow Tie” car builders who want to keep their hot rod or street rod “all Chevy.” The Muncie gearboxes are also easy to adapt to a wide range of rods based on other GM models and even non-GM rods. There are three types of Muncies: M20, M21 and M22.
The Muncie four-speeds came in many GM models of the 1960s and 1970s. They were used in many muscle cars and Corvettes, while milder GM cars employed Saginaw or Borg-Warner four-speeds. A Muncie transmission differs from a Saginaw transmission in that its reverse lever is in the tail housing, not the side cover. The main difference between a Muncie and a Borg-Warner is that the Muncie has a 7-bolt side cover (two less bolts than a Borg-Warner unit).
Many Muncie main cases, tail housings and side covers have casting date codes. The code consists of two ½-inch 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 which indicate the week of the month. The status marker was used at the Muncie, Indiana 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 fairly rare.
A serial number is stamped on all Muncie transmission cases, which has nine symbols, w. The first symbol indicates the division, the second matches the last digit of the 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.
Before installing a Muncie in your hot rod or street rod, it would be a good idea to check the serial numbers and other codes with a transmission expert. You may have found a hard-to-get Camaro, Corvette, GTO, 4-4-2 or Buick GS four-speed tranny which is worth a lot of money to a purist. You could probably swap one of these for a better unit and cash to boot. Finding the right box in a junkyard could turn out to be like winning the lottery.
David W. West of David’s 4 Speeds, LLC is a specialist who rebuilds “Big 3” four-speed transmissions and who can help identify what you have. About 95 percent of West’s work involves GM Muncie units. According to this transmission guru, all Muncies work on basically the same principle. The input shaft goes to the cluster gear. The cluster is mated to all the four speed gears, which are independent of the main shaft.
“For the whole thing to work, the magic of the Muncie or the four-speed is in the synchro assemblies,” says West. “They are the key to the whole thing.” West uses a drawing to show how 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. “That’s the whole shifting process,” West says. “And, of course, the downshifting going into the next gear.”
West tells people to think of a transmission in terms of levers. The lever is engine power, which 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 of an engine.
West specializes in concours-quality transmission “restorations,” rather than standard rebuilds. His customers have huge investments in their cars and want their transmissions done to concours standard. They are interested in details like specific finishes and re-plated original poles. In reality, the transmissions in these cars are overdone. “They were never done that good to begin with,” says West. “But, many of my customers feel much more comfortable with good quality stuff that has all the right factory codes and date codes.”
West was a heavy equipment mechanic in a factory for 30 years. He worked on everything from 25-ton fork trucks to Cushman scooters. “I did a lot of transmissions and it was the same in principle,” he says. “The big difference was I used a 10-ton crane to work on those and the Muncies I can lift by hand.”
West had a 1966 Chevelle SS. The fellow who sold it to him in 1970 put $1,100 into the motor. “That was huge money in 1970 and he just wanted to get out of it for $1,100,” West recalls. “My dad took it for a ride and drove it and said, ‘There’s no way you’re going to drive this thing on the street.’ He told me he’d sign for it, but that I could do drag racing with it only. I thought that was cool.”
At Great Lake Dragaway in Union Grove, Wisconsin, the car’s first run was an 11.56 — an incredible speed back then. Hooked on drag racing, West kept doing it for several years, until1974, when he went to work in the factory. He retired in 2004 and started David’s 4 Speeds, LLC to do transmissions full time. He was rebuilding four-speeds as a hobby as early as 1990.
A Corvette collector car dealership in the Milwaukee area was having trouble getting quality transmission work done. The owner asked West to do a four-speed for him. He was impressed with the job and offered to have West do his whole inventory. People from all over started calling, and it snowballed from there.
Much of the West’s transmission work involves customers wanting something specific for their car. A customer may have a 1969 Camaro ZL1 but wants a correct, dated transmission. Most of the core transmission cases he buys now are dated. He tries to harvest just the cases. Most of them are made before the1966-1970 period and were abused. He buys them hoping to just get good cases out of them, then rebuilds them using all new gears and internal components. West doesn’t get involved in manufacturing parts. With some of the good aftermarket products offered now, he can build a transmission just out of parts.
The Muncie M20 is basically a wide-ratio transmission and was intended for normal street driving use. The early models 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, 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 well in street 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 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 often say. People think all Muncie four-speeds are Rock Crushers, but they’re not.
The M20 and M21 came out about the same time in 1963, but the M22 didn’t come out until 1965. It was developed for the Corvette Grand Sport racing program. Only about 50 early units were made. There were very few M22s until 1969. A lot of the later M22s were service replacement units sold over the parts counter and installed by dealers. They have stamped coding on them.
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. Most of West’s customers want factory original transmissions, but beefed up boxes for hot rods and resto mods are popular, also. “The Barrett-Jackson stuff tends to be over restored,” says West. “Take different finishes, for example. The norm is the rear case is painted with a clear coat on it. Some people put the case in a vibratory finisher that gives it a different look. Some cases are plated. They look like nickel looks when hit with tin. They take on a duller chrome appearance, similar to something they’d use in the manufacturing of chrome.”
For hot rodders and street rodders, the type of finish is up to personal preference. “It’s a matter of whose perception is what,” says West. “At the factory, the main case was kind of shiny silver and the tail housing tended to be a different alloy with a duller gray finish.
When rebuilding a Muncie four-speed (or other unit), West can often compensate for OEM problems, which were more pronounced in the earlier Muncie transmissions. According to West, the 1963 unit was “kind of a wreck.” The 1964 and 1965 transmissions are very popular, especially with Corvette enthusiasts. Due to the values of these cars, the owners usually want an original appearance.
West can take a 1965 case and put in later model stuff. He can bore the cluster pin and put in a one-inch pin or put in a later synchro assembly to give his customers the functionality of a later model transmission but the looks and aesthetics of a 1965. It doesn’t cost much more to have this upgrade made if West is doing the transmission anyway.
If you’re thinking you can save money by getting a cheap build for a hot rod, guess again. West doesn’t don’t do transmission work to different levels. He looks at this as art. “If a guy wants less quality, I refer him elsewhere,” he says.
According to West, synthetic oils shouldn’t be used in Muncie four-speed transmissions, no matter what type of car they are in. He feels synthetics are not “synchro friendly.” Because synthetics are slippery, they don’t allow synchro rings to grab on to the cone of the gear and synchronize properly, which 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, which causes the synthetics to leak out of them more easily than natural gear lubes.
A GL4 is West’s gear lube of choice — a natural 85/90W gear oil. GL5 is supposed to supercede GL4, but there’s lots of discussion about this among hobbyists. GL5 has not been around long enough to know how it’s going to unfold, so West recommends being safe rather than sorry. He thinks GL5 has sulfur in it, which is corrosive to brass. It also acts like synthetic and doesn’t allow synchro rings to grab the cones of the gears. West suggests sticking to a GL4.
If you’re thinking of putting some kind of exotic shifter in your hot rod and topping it off with a beer tap, you might want to think again. According to West, one of the main transmission issues is shifter problems. Grit that gets in the grease of the shifter mechanism can cause real problems. People also fail realize how important shifter geometry is.
Each shifter is specific to its application. A shifter handle has a specific bend to it. This can be important, especially on the top side, where the gearshift has to clear a console, a custom dashboard, a bench seat, or even too hard a shifter boot. According to West, “Anything that impedes the gear shifter’s movement can keep a shifter from going in and going as far as it has to go.”
Shifters also have three selector plates, which come out of the bottom and have a unique bend to them. The unique length corresponds with the arm that’s attached to the shift shaft. The rods are also 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.
If there are sloppy arms or arms too long in length, it could allow easy shifting into one gear, say third, but not allow the same shifter to go all the way into fourth. Many times, a hot rodder will say his transmission is grinding when it goes into fourth gear, but in reality, shifter geometry is the problem.
West works chiefly by mail order. Most transmissions he rebuilds are sent to his shop in the Pewaukee suburb of “Brew Town.” He gets shipments from all over the country. A lot of people call him looking for a correct transmission. About a third of his business comes from customers ordering units which are already complete.
West also does Ford and Mopar four-speeds, but no imported jobs. It’s pretty much centered on trannies made by the Big 3 automakers. He also avoids commercial accounts. “I try to keep it small,” he says. “I don’t really want to grow into a big corporation. I enjoy what I am doing. I like to have hands on with my customers and be a part of that whole bond. I’m the one that actually rebuilds them, so I guarantee them. I know they’ve been done to the best of my ability.”
A quality rebuilt M20 is worth about $1,000–$1,200 on the open market. Rebuilding one in your home shop is a complex process. Books are available which provide detailed illustrations and parts breakdowns. Go slow and take your time. Work in a clean environment and use lint-free rags. Keep all parts and your hands free of dirt and even dust. Clean your workbench before you begin any assembly. If you walk away from the job for long periods, cover all parts on the bench with clean, clear plastic sheets.
David’s 4 Speeds, LLC
Aftermarket suppliers like Auto Gear make quality parts that can be used to rebuild or improve original GM Muncie four-speeds.
This cast iron mid plate is used to stiffen the transmission assembly.
The top row shows a synchro ring on left and a cleaned up reverse gear on the right. Bottom left is an improved purged brass synchro ring. Next to it is a regular and less expensive brass synchronizer ring. On the right, a reverse gear before being cleaned up.
A later-style cover with the clevis pin in the middle.
A synchro hub and a pair of cluster pins for a Muncie four-speed.
This cutaway Muncie transmission shows a good look at the main shaft on top and the cluster gears on the bottom.
Dave West uses this colorful drawing to show the workings of a Muncie four-speed gearbox.
The upper row shows three different shift shafts used over the years. Three synchro rings are stacked vertically on the right. A later-type bearing retainer is on the bottom, in the center. Next to it on the left is a synchro hub.
This is a bearing retainer for an early 1963 Corvette. It is small and made out of aluminum, and it was quickly made obsolete by GM. Originals are hard to find.
The tail housing is cast of a different alloy that has a darker gray color.
This Muncie main case carries specific factory coding.
The synchro hub is an important part of the synchro assembly in a Muncie four-speed transmission.
The later Muncie gearboxes used a much sturdier bearing retainer which holds up better in service.
These are two of the different type shift shafts used in Muncies over the years.
“For the whole thing to work, the magic of the Muncie or the four-speed is in the synchro assemblies,” says Dave West. These are synchro rings.
This heavy-duty sealed bearing is an example of improved aftermarket parts. It is designed for use with the Super Case made by Auto Gear.
This is a Muncie bearing that is similar to the original with 12 ball bearings.
The length and shape of the shifter plays a critical role in shifter geometry and in clearing obstacles inside the car.
The linkage must work smoothly, without any type of binding.
This 1963 Muncie case is a valuable item, even though it wasn’t the best design.
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