Tapping Chart: NPT National Pipe Taper Threads
Nominal
OD + TPI		 NPT & NPTF NPS
1/16-27 D 1/4
1/8-27 R S
1/4-18 7/16 29/64
3/8-18 9/16 19/32
1/2-14 45/64 47/64
3/4-14 29/32 15/16
1-11-1/2 1-5/32 1-3/16