Took the belt pulley housing off a locked-up M and found a needle bearing from the transmission pilot bearing in the constant mesh gear. Both the constant mesh driving and driven gears have teeth missing.
I have a junk Super M, can anybody confirm that the transmission input shaft and constant mesh gears will interchange with an M? I don't want to pull the Super M apart if somebody with a parts book or first-hand knowledge can't confirm they will match.
This is one of my best M's, belonged to my grandpa, and I use it for custom baling all summer. I think the Super M rear end would fit on the M, but I'd really like to keep the major castings together.
By they way, the symptom of impending trouble was noise in second gear, only under heavy load, not just towing an implement. I worked it hard in third for six weeks every day after the noise in second started, and stayed away from second. The needle came out and started breaking teeth as the tractor sat at a fast idle while I was hitching an implement (maximum use of the pilot bearing). I could hear a few clicks. I shut it down, and the transmission was locked up tight from that point on, with a needle stuck between two teeth.
I should have stopped using the tractor with the sudden appearance of second-gear noise, but you get lulled into the excellent reliability of these tractors, including those that make transmission noise for years on end with no apparent problems.