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Shop now for Parts and Manuals for 1960-1966 Chevy and GMC trucks with SM420 Muncie 4 speed transmissions. (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 1960-1966 model years the SM420 was the only 4 speed manual transmission used in the 4x4 models. The 4x4 and 4x2 transmission are the same for these years, in fact, GM lists the same transmission assembly part number (3917771) to fit 10 through 30 series with 4 speed manual transmissions from 1948-1967!
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 4x4 engine and transmission conversions due to its short length and 7.05:1 1st gear ratio.
