Epilogue:
(Posted for the benefit of others)
As it turned out, it WAS a 3/8" fine thread- You were exactly correct delray!:thumbsup: I would still like to know if this was the standard for an HS50, instead of the usual 5/16" fine, as I have a "suspicion" that my engine may have originally been a 1" shaft that was lathed and re-keyed down to 3/4".:grind: