If it was me, I'd call Peter up at Nisonger and ask him just what the deal is with the funky thread. I know he sells his Smiths stuff with your choice of either 5/8 UNF or 3/8 NPT temperature sensor fittings... so it just might make a difference. Certainly worth the price of call before you start carving.
Orrrrr, you could do like the rest of us and just jam the threads in good crossways, use a lot of leak-b-gone goo, and torque it down like crazy.
