bartamos,
You are correct about the straight SAE 1/2-20 threads! I just went out to the garage and found a 1/2-20 hex nut. Bingo! You are also correct in how a flared tube seals. I do want to get a couple of barbed brass hose fittings that accept a flared tube fitting, so now at least you helped me figure out the correct type of threads I need to specify. There is a Fastenal here in town and I can order these parts from them. Ordered a new shut off gas tank valve and hope it fits. Hard to tell right now about the NPT threads in the tank outlet, but I think your guess is probably correct.
Want to solve all gasoline leaking issues! Right now its the tank valve that is giving me issues.
