The Stovebolt Site Forums The Shop Area Driveline Muncie SM420 transmission question

While I was getting this old transmission freed up to shift smoothly from sitting 30+ years out of my 48 COE, I had the top tower off moving the 3rd and 4th gears by hand I somehow slid it too far or something and this little dog or part (see it on the picture) got out of its position. Is there a trick to get it back in place without pulling it all apart.? Also, this transmission used to jump out of 2nd gear for years, we always had to hold a little pressure on the shift when slacking off on the gas or it would pop out. I wanted to see if I could find a reason for this too while it's apart, Any ideas will be appreciated. Thanks

popping out of gear is usually due to a worn bearing. I had that problem on two different SM420s. One popped out of 3rd on decel and the cause was a worn clutch-gear bearing. The other popped out of 2nd on decel and the cause was a worn output shaft bearing.

Given your situation I would prepare to pull the trans apart and fix both problems. I would order bearings from one of the bearing houses rather than buy a kit. I think you'll get better quality bearings that way. Also check the u-joint yolk for loosness on the splines and a groove from the seal. Use a speedy sleeve to fix the latter. Replace the yoke to fix the former.

The other thing to check is the pilot bushing in the flywheel. That maintains the alignment of the input shaft. A worn pilot bushing can cause a transmission to pop out of direct (4th in the case of a 4-speed.)

That part looks like a clutch key. See part #35 in the exploded diagram in this attached file. I bought the BK108 kit when I rebuilt my SM420. Bearings seemed good to me; made in Japan if I recall correctly.

You might want to look for a newer transmission. I don't think the brass synchroniser rings are available for the early ones - I was able to get the them for my 1967 transmission. Bonus was the 1967 transmission came with backup light switch.

Thanks for the ideas. I did figure out how to put the little dog part back in. Nothing to it once I figured out the way it all works. I just inserted a narrow strip of an old plastic credit card above the spring loaded rings and put the little dog on top and it slid right in. Then pulled out the plastic.

I'll check out the bearings and bushing, but it always stayed in the high gears, and super low, just 2nd was the problem.

Thanks for the diagrams and info. I do have another transmission out of a 1960 Viking 2 ton. It shifted great too, we took engine and the transmission out before it was scrapped. It's the same except it has the E brake wheel on the output end. I was removing all that but discovered that the cast bracket that holds the brake is also the lower bearing housing. Now I'm wondering if I can switch out that part for the bearing housing on the old 48 transmission and I'd have to use the COE tower as well to be able to shift. I may not know until I try switching the housings..

Yes, you can change out the rear bearing retainer.

For jumping out of gear, check the detent ball spring. Mine was broken and would jump out of gear in 4th going down hill.
Easy fix if that’s what it is.