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Shop now for Parts and Manuals for 1948-1959 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. The SM420 was used from 1948-1959 as the only 4 speed offered for most 1/2 ton to 1 ton models, 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.
