I thought a too-small key would break or chew up the keyway and it would have to be absolutely tiny to actually escape from under the gear, but I haven't tried it lol.
As blindfolded mechanics we can't assume that the engine ever had a screw holding the gear on.
I can't get the gear to seat flush. I've tried everything. Including using some wood and hitting it with a hammer.Yeah unless his screw came off the shaft It either partially sheared and started grinding or idk what. All I know is if there's too much slop in the key way it will start "hammering" and won't hold. I think he said it happens when he engaged the clutch meaning he's releasing the lever pretty fast probably. He also said the small gear was spinning freely, it should spin it's connected to the whole rotating mass, crank, rod, piston. Maybe he isn't tightening the screw enough for the gear? sometimes the gears are tricky as they should ideally be flush with the clutch wheel but on most engines they aren't fully seated from factory.