The 3rd gear synchro on my 64 SM-420 is a little weak. But then I remember back when those were new trannies and they were never made for speed-shifting. I learned how to double-clutch on the 420 and whatever its crashbox predecessor was. I drove ice cream trucks when I was a teenager and they were invariably Advanced Design chevys or Jimmys. They all had 4-speed crashbox transmissions and required a double-clutch technique to get from gear to gear without removing the teeth.
I got so that I could do it without the clutch. You find lots of things to do when you're bored out of your skull listening to that silly-arsed ice cream music for 10-14 hours a day.
But the lessons were invaulable. I still double-clutch up into 4th and down into 3rd...just cuz the 420 seems to be happier that way. And yeah, the clutch is heavy as hell.
More "I remember" stuff. When I was in high school, I took autobody classes in the summertime. Some guy brought in a brand new Chevy truck (must have been around 62 or so) that had a ding in the fender. He had all of us try to press the clutch pedal down. There were only a few kids who could do it. I depressed it..but only by using both legs.
It was one of the stiffest clutch pedals I think I've ever seen.
