C3 Corvette Transmission Codes 1968-1982

Reading your transmission's build code

Every C3 left the factory with either a manual gearbox or one of Chevrolet's automatics, and each carries its own stamped or riveted code. Read it and you can confirm what the car was actually built with, which is the difference between a matching car and one that has quietly been given a replacement box. The tables below cover every model year from 1968 to 1982.

Where to look depends on which gearbox you have. On the Muncie four speeds of 1968 to 1974 the stamp is on the passenger side of the case, on a machined boss just ahead of the tailhousing. On the Borg-Warner Super T-10 that replaced it the pad moved to the driver's side, high at the rear near the side cover, so if you are feeling for it on the wrong side you will not find it. The Turbo Hydra-Matic 400 carries a riveted tag on the passenger side above the pan rail, plus a partial VIN stamped on the driver's side, the same convention used on the engine block. On the Turbo Hydra-Matic 350 the tag sits on the right hand face near the pan, and it is one of the most commonly missing identification points on a C3, so do not assume the worst if yours has gone.

The Muncie month letters

The Muncie stamp is the best documented of the four and the easiest to read once you know that the month is a letter, and that the alphabet skips eight letters along the way, F, G, I, J, L, N, O and Q:

From 1969 the Muncie stamp ends with a letter giving the ratio, and it is the same letter you will find in the tables below: A for the wide ratio M20, B for the close ratio M21, and C for the M22, the one everybody calls the Rock Crusher. That last character on the case and the code in our table are the same thing.

We have deliberately not printed a worked example for the Borg-Warner Super T-10 or the Turbo Hydra-Matic 350. The published descriptions of those two disagree with each other on the order of the elements, and we would rather leave a gap than print something that sends you looking for the wrong character. If you have a clear photograph of either stamping on a car whose history you know, the contact page is open.