Lock washer will cure it, nylock nuts will just melt the nylon out and not hold.

3/8" - 16 is what you want, or 3/8" -24 depending on the stud. Fine or course thread, pretty easy to see the difference. Also make sure the donut gasket is not smashed if it has one. My 250 uses a gasket and once they start to smash, it just keeps going and the pipe stays loose.

Joe