I am still going for the story that the acorn nut can unscrew with the starter spinning it off. I have seen it many times. Even tightening it pretty hard. I guess the guys I saw just did it over the nut, and there was not enough friction to hold it tight. The prop will likely not spin it off on it's own I doubt. Hopefully the OP will have good luck with his, and I would like to welcome him to the forum. I am a retired toolmaker/machinist too, and have read a fair bit of that book looking for oddball thread specs that we always seemed to need. Pretty thick and expensive book, pretty dry read but informative.