I had this problem with an old Saito 40. Drove me batty because the nut on the engine fit, but nothing else would. Inspection with a magnifying glass revealed a small flaw; after a light touch with a file the problem was solved. The nut on the engine must have had a tiny bit more thread clearance than the other nuts. But the flaw was at the end as Don says and the nut would not go three turns. Still, after all this frustration I would certainly get a lens and examine it. Jim