During the next-to-last snowfall, the gearshift on our MTD snowthrower jammed in high gear, but the wheels turned much more slowly than usual. Slightly before the last snowfall, I removed the cover over the transmission and discovered what went wrong:
That rubber wheel should be resting on the circular transmission plate, but somehow it slid off the far right edge. The spring-loaded clutch cable then pulled the plate upward, so that the side of the wheel drove the edge of the plate. Ouch.
The plate rotates on a bearing around a post on the folded red steel support structure underneath it, which pivots on a rod across the transmission housing behind everything that’s visible here. That rod used to protrude through the housing, but it had slipped inside and moved the plate to the left enough to let the wheel fall off. Some awkward maneuvering got it back through the hole, which made the real problem obvious:
There’s supposed to be a cotter pin or hitch pin clip through that hole, with a washer matching the obvious wear marks:
That’s actually a spacing shim from a collection that I’ve used rather infrequently over the years, but it’s exactly the right thickness to make the answer come out right.
A few weeks later, we found the missing washer on the driveway at about the point where I first noticed the transmission wasn’t working. It’s in the box of parts, waiting for the new cotter pin to wear out.