1967 GMC SM420 Transmission Parts

Shop now for Parts and Manuals for 1967 Chevy and GMC trucks with the SM420 Muncie 4 speed transmission. (SM = Synchro Mesh)

The Muncie SM420 4 Speed Manual Transmission is a very compact, heavy-duty 4 speed manual transmission that is synchronized in 2nd, 3rd, and 4th gears. For the 1967 model year, the SM420 was the only 4 speed manual transmission used in the 4x4 models and the primary 4 speed used in 4x2 trucks. Unlike earlier years, the 4x4 and 4x2 transmissions are the different for 1967 as the transfer case is directly mounted to the back of the transmission for 1967 4x4s.

The SM420 has a transmission case length that is less than 10-1/2". The compact design was made possible by using the same part for mainshaft 1st gear, mainshaft reverse gear, and 1-2 synchro slider. The design is robust enough it was also used on Class 6 Two Ton trucks and it is highly sought after for vintage Jeep 4x4 engine and transmission conversions due to its short length and 7.05:1 1st gear ratio.

The SM420 was replaced by the somewhat larger and stronger SM465 4 speed for the 1968 model year.