TruThread P NPSF
Thread plug gauge NPSF

ASME B1.20.3
NPSF threads (National Pipe Straight Fuel) are straight pipe threads designed to provide a seal without additional sealants when used with the correct mating part. They are specifically intended for use with NPTF (National Pipe Taper Fuel) external threads to create a mechanical seal.
Unlike NPTF threads, NPSF threads are cylindrical, but they are dimensioned in a way that allows them to form a tight seal with tapered NPTF threads. These threads are commonly used in fuel systems, hydraulic applications, and other high-pressure systems where a reliable seal is required without using sealing compounds.