National pipe straight (NPS) taps produce a straight pipe thread for low pressure applications. The thread produced will require the use of a thread sealant to ensure a tight seal
National pipe straight fuel (NPSF) "Dryseal" taps produce a straight pipe thread for low pressure applications. These taps produce a tight seal without the use of thread sealants
Pipe taps are the nominal size of the pipe fitting and not the actual size of the tap