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FEATURE ARTICLES

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.
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.
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.
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."

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