Tim,
My SM420 (4-speed, granny gear, right?) would grind with a reasonably brisk single clutching. Doubling clutching solved the problem but just pausing an instant in neutral on the way to high gear also works so that's what I do now.
1948 3/4-Ton 5-Window Flatbed Chevrolet 34 Years. Now with a '61 261, 848 head, Rochester Monojet carb, SM420 4-speed, 4.10 rear, dual reservoir MC, Bendix up front, 235/85R16 tires, 12-volt w/alternator, electric wipers and a modern radio in the glove box.
