OK, I'll take a shot at describing the technique of double clutching. I've got the same crashbox 4-speed in my '46 - been driving it for 28 years. Ex long-haul trucker as well.
It's correct that you're basically trying match the speed of the gears before making them mesh. On an upshift (1st to 2nd for example - which is really 2nd to 3rd since you don't use "granny 1st" for driving), you run the truck up to its shift point in 2nd, clutch in, shift to neutral, clutch out. Now when the engine spins down to the RPM it will be running when engaged in 3rd gear, clutch in and shift to third. Upshifts are easier then downshifts. 3rd to 4th is easier than 2nd to 3rd.
On a downshift run the truck to the point it needs a lower gear, clutch in, shift to neutral, clutch out. On downshifts you need to spin the engine up to the RPM it will be running when engaged in the lower gear. Rev the engine up to this point, clutch in and downshift. On an over-the-road diesel you can use the tach to figure your shift point, but that's not an option on our older trucks. It's a touchy-feely thing. It seems impossible at first, but it's like anything else - pretty simple when you get the hang of it. That said, I still miss an occasional downshift and grind gears.
The good news is that those "gear jammer," "rock crusher," "crash box" gearboxes(pick your favorite term) are pretty darned stout. That same 4-speed was used in WWII in the 1 1/2 ton truck (looked like a deuce and a quarter, but with 6 wheels instead of 10). Whole generations of farm boys learned to drive on them and had a hard time tearing them up. That trans is super heavy duty in a 1/2 or 3/4 ton truck - it can take a heck of a beating and come back for more.
Hope that helps,
David
Last edited by David Bush; 10/29/2014 3:54 PM.
