The SM-420 is a solid 4-speed that can handle the 383. It will give you granny gear when you really need to pull out of a hole with a heavy load and has a nice spread between all of the gear ratios. Final gear (high Gear) is 1:1 just like the TH-350/400 and everyother non-OD manual transmission. This trans was also used in the 2-1/2 ton trucks rated at 19,000 GVW. Under normal driving conditions you'll drive it like a 3-speed (2, 3 & 4).
Low gear is not synchronized, all others are...
Mike B
