My truck has an SM420 4-speed. It was loud (yes, even louder than a normal 4-speed) in gears 1-3, and it was not synchronizing properly shifting into 2nd. So I pulled it apart today.
It looks like at some point in the past, the 2nd gear synchro retainer ring broke, and the pieces got lunched in the gears. Most of the pieces were in the bottom of the trans, buried in gunk, so I think this happened some time ago. The 2nd gear synchro was kind of flopping around in there when I looked inside. The retaining ring also ended up taking a pretty big chip out of one of the gears on the countershaft:
[img]
http://rhoville.com/mattsnaps/sm420-chip.jpg[/img]
I figure if I rebuild this transmission and put a new snap ring in, that will probably solve the synchronization problems I've been having. But thanks to the really big chip in that gear, I also figure it is going to be loud forever. It doesn't pop out of gear or anything.
So here's my question for the group: If you had a similar transmission, would you put a rebuild kit in it, or would you take this as a sign from above that it's time for a 5-speed or automatic trans swap?
Follow-up question: Anyone got a countershaft assembly for sale?
