I rebuilt a couple of these transmissions years ago when I was a kid. They're fairly simple (once you get past the weird enclosed U-joint setup). As I recall the main drive gear would wear and cause problems with the synchro rings because either and/or the front bearing in the transmission had too much clearance or, in some cases, the pilot bearing in the crank was worn and allowed the splined shaft the clutch plate was on to jump around. Kind of a progressive failure, so to speak.
