The Magical Muncie 4-Speed Transmission
By John Gunnell
Muncie four-speed manual transmissions came in many GM models of the '60s and '70s. They were used in many muscle cars and Corvettes. Milder GM cars used 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 has).
Many Muncie main cases, tail housings and side covers have casting date codes. The code consists of two 1/2-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, Ind., plant to keep track of problems and is generally not important to enthusiasts.
This Muncie main case carries specific factory coding.
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. Some Camaro, Corvette, GTO, 4-4-2 and Buick GS four-speed Muncie transmissions are very rare and worth a lot of money to a purist.
A later style cover with the clevis pin in the middle.
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 the 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 speed gears, first, second third and fourth. The speed gears 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."
Dave West uses this colorful drawing to show the workings of a Muncie four-speed gearbox.
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.
West actually 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."
The tail housing is cast of a different alloy that has a darker gray color.
West was a heavy equipment mechanic in a factory for 30 years. He worked on everything from 25-ton fork trucks down to Cushman scooters. West had a '66 Chevelle SS that got him hooked on drag racing. When he retired from his job in 2004, he started Davids 4 Speeds LLC to do transmissions full time. He had been rebuilding four-speeds, as a hobby, since as early as 1990.
Much of the West's transmission work involves guys wanting something specific for their car. A customer may have a '69 Camaro ZL1 and want a correct, dated transmission. West will build it for him or her. Most of the core transmission case 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 good cases out of them. He rebuilds them using all new gears and internal components. West doesn't get involved in manufacturing parts. With some of the good aftermarket stuff offered now, he can build a transmission just out of parts.
This cutaway Muncie transmission shows a good look at the main shaft on top and the cluster gears on the bottom.
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 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.
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. The M21 was for higher performance cars. 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 straight-cut gears. It uses much the same gear ratios as the close-ratio M-21, but can take more abuse. An M22 retains less heat due to its straight-cut gears and the lack of thrust that helical-cut gears create. An M-22 holds up well. The downside is it tends to make noise. It will rattle at low rpms and whine at high rpms. The M22 is called the Rock Crusher. 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 and a lot of the later M22s were service replacement units sold over the parts counter and installed by dealers. These have stamped coding on them.
Upper row from left starts with three different shift shafts used over the years. Three synchro rings are stacked vertically. A later type bearing retainer is on the bottom, in the center. Next to it on the left is a synchro hub.
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, too.
"Barrett-Jackson stuff tends to be over restored," says West. "Take the finish. 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. It's a matter of whose perception because the factory main case was kind of shiny silver. The tail housing tended to be a different alloy with a duller gray finish."
The synchro hub is an important part of the synchro assembly in a Muncie four-speed transmission.
When rebuilding a Muncie four-speed (or other unit) West can often compensate for OEM problems, that were more pronounced in the earlier Muncie transmissions. The '63 unit was "kind of a wreck" west says. The '64 and '65 transmissions are very popular, especially with Corvette guys. Due to the values of these cars, the owners usually want an original appearance.
West can take a '65 case and put in later model stuff. He can bore the cluster pin and put in a one-inch pin. He can put in a later synchro assembly to give his customers the functionality of a later model transmission and the looks and aesthetics of a '65. It really doesn't cost much more to have this upgrade made if West is doing the transmission anyway. West doesn't work to different levels. "If a guy wants less quality, I refer him elsewhere," he says.
The later Muncie gearboxes used a much sturdier bearing retainer which holds up better in service.
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 that 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.
"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.
A GL4 is West's gear lube of choice — a natural 85/90W gear oil. GL5 is supposed to be superceding GL4, but there's lots of talk about this in the car hobby. GL5 has not been around as 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 that 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
According to West, 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.
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, when 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 that come out the bottom that have a unique bend to them and unique length that 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 changes and affect the shifting geometry. Having the proper geometry is a necessity.
"The length and shape of the shifter plays a critical role in shifter geometry and in clearing obstacles inside the car.
If there are sloppy arms or arms too long in length, it could allow shifting into one gear, say third, very positively, 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 is from customers ordering units that are already done.
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 that 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 on it. If you walk away from the job for long periods, cover all parts on the bench with clean, clear plastic sheets.. Keep Dave West in mind. He buys sells and exchanges four-speed transmissions. You can contact him at Davids 4 Speeds LLC, 315 Jonathan Dr., Pewaukee, WI 53072. Ph: (262) 513-8331. Email: David@Davids4Speeds.com. Website: www.Davids4Speeds.com.