đŸ”— PDF datasheet thread types
Thread types: what they are, how they seal, and why it matters
A thread is a helical coupling machined onto the outside of a cylindrical or conical element (the male fitting) or onto the inside of a component (the female fitting)
Joining the two creates a mechanical bond that can be assembled and disassembled.
The connection is either held tight by friction or reinforced with a physical barrier: sealants, gaskets, or washers.
For marine hydraulic applications, four thread standards are relevant: BSPP, BSPT, NPS and NPT.
Geographic scope
NPT and NPS are North American standards (United States and Canada). BSP — which includes both BSPP and BSPT — is the reference standard across the United Kingdom, Europe, Asia, Australia, New Zealand, South Africa, and most of the rest of the world.
BSPP – British Standard Pipe Parallel
BSPP is a cylindrical, parallel thread, also referred to as a GAS thread. Because the male and female profiles run parallel and never compress against each other, the thread geometry alone does not produce a seal. Sealing requires an O-ring, washer, or sealant placed between the fittings and compressed when the joint is tightened.
BSPT – British Standard Pipe Tapered
BSPT is the tapered variant of the BSP family. The seal comes from thread deformation: as the male is driven into the female, the tapered profiles compress against each other and close off the joint. The thread angle is 55°. The tapered geometry produces a seal on its own, but the use of a thread sealant (tape or compound) is still recommended practice.
BSPT and NPT look similar at a glance, and the fittings can physically be screwed together, but the joint will not seal. The 5° difference in thread angle (55° vs 60°) prevents the profiles from mating correctly, leaving gaps that will leak under pressure.
NPT – National Pipe Thread
NPT is the American standard, compliant with ANSI. After BSP, it is the most widely used pipe thread standard in the world. The thread is tapered, with an angle of 60°. Sealing occurs through contact and compression between the male taper and the female taper as the joint is tightened. The geometry produces a relatively tight seal on its own, though the use of a thread sealant (tape or compound) is still recommended practice.
NPS – National Pipe Straight
NPS uses the same 60° thread angle as NPT but in a cylindrical, parallel form. The thread geometry does not seal on its own. Sealing requires an O-ring.
Why the distinction matters
The critical difference between BSP and NPT is not just geographic. The two systems are not interchangeable: NPT threads have a pointed profile, BSP threads have a rounded one. The thread angles differ by 5°. Forcing fittings from different standards together may allow assembly but will not produce a reliable seal — and under pressure, that matters.
Choosing the correct thread type for each application is a prerequisite for a leak-free, pressure-safe installation.