NPT Pipe Fittings

NPT is the one common hydraulic joint whose thread and seal are the same feature. Male and female tapers wedge together as the joint is made up, and the interference across flanks, crests and roots holds the pressure. Every further turn changes both how deep the fitting sits and how well it seals.

Engineers keep specifying it because nothing else is this cheap, this widely stocked or this tolerant of grit. ASME B1.20.1 covers NPT, which normally wants PTFE tape or an anaerobic sealant, while the NPTF dryseal variant tightens crest and root tolerances so the threads crush into contact unaided. The cost is inherent to a thread seal: it backs off under vibration, an elbow ends up facing wherever the last turn leaves it, and every remake cuts more thread.

Where it's specified

• Static, low-duty plumbing — nothing shakes the joint loose, so a thread seal is adequate and costs least
• Gauge, drain and fill points — small connections that never justify a machined port
• Tie-ins to existing pipework — matches the installed base of pipe, nipples and cast ports
• Field and repair work — threads can be recut on site, and there is no soft seal to damage

Range and materials. Straights, elbows, tees, hex and close nipples, couplings, bushings, caps and plugs, plus adapters converting pipe to JIC 37°, ORFS, SAE ORB, BSP and metric, from 1/8 in. through 2 in. Zinc-plated carbon steel, 316 stainless and brass all run through the range, with brass favored on instruments.

Ordering notes. Use tape or sealant on plain NPT, starting two threads back so none of it washes downstream; dryseal into dryseal is the only pairing meant to seal bare. Count turns past finger tight rather than chasing torque, and stop when the fitting points where it should — never back it off to aim it. Where vibration or precise clocking matters, an ORB or ORFS port is better.