Milton (S-684) 1/8" MNPT Male Tank Valve 1/8" MNPT tank valves are designed for use in filling vessels with air. Standard valve core with plastic cap measures at 1" overall length. Maximum pressure of 200 PSI. Cap thread size of .305-32. Other recommended tank valve size: Milton #S-684-4 (1/4” male NPT, 1.5” overall length w/cap) tank valve.