dont know the part number of the new nut I have,it was given to me by the shop that is repairing the stainless prop, however, I did a cross reference on the part listed in the parts manual, it is 11-52707 which cross referenced to a Siera 18-3700, so I ordered one on line.. It is a 3/4 nut, but not sure about the number of threads, I saw one listing that said that nut was !6, and another said it was 20. I considered getting a 3/4 - 16 die, but wanted to make sure the prop shaft was 16, which I Think is considered "fine". I didnt want to put the die on the wrong threads, thats why I was wondering if Mercury used different threads. The prop nut went on maybe 4 or 5 turns, but it was hard so I stopped. This prop hasnt been off in quite a while. I still have the original aluminum prop, but have had a stainless steel on it for the past 35 years, which brings up another question, I have two Front thrust washer, part #12-835467-5, and 77897. The 77897 is about 1/4 inch thicker. everything with the original 12-835467-5, but I have been told it was too thin. The prop has a 1/2 inch lip and on the end that goes into the housing. It goes in about 1/4 inch leaving 1/4 inch.I might add, that while this boat is 38 years old, it only has 390 hours on it, so obviously it doesnt run a lot. This all started when I hit something under water and I had a mechanic replace drive shaft, lower shift cable and other things just because of age. By the way,from an appearance standpoint, the threads on the prop shaft appear to be ok. . So should I assume that everything has been working to just put it back like it was when I get the stainless prop back?