NPT vs BSPT vs Straight Thread: Which Pipe Thread Are You Actually Working With?

Jump to what you need

Quick Answer

Use NPT on US pipe, fittings, and most North American air, water, and fuel systems that say “pipe thread.” It seals on the taper (plus sealant).

Use BSPT (R) on British/ISO tapered pipe (ISO 7). It also seals on a taper, but 55° and different diameters — it is not a substitute for NPT even when TPI matches.

Use straight/parallel (BSPP/G, NPSM) when the print calls a seating face and a washer or O-ring. The threads only locate. Tape on a parallel thread jammed into NPT is not a design.

NPT — US Tapered Pipe Thread

ASME B1.20.1. 60° thread, 3/4" per foot taper, nominal sizes that do not match the measured OD. Hand-tight L1 plus wrench-makeup to L2 is the whole game. North American black iron, brass fittings, compressor drops, and most US hydraulics adapters that say “pipe” are NPT (or NPTF dryseal — do not casually interchange gauges).

Size it with the NPT calculator and the sizing guide. Do not use the UNC tap-drill chart.

BSPT — British Tapered Pipe Thread

ISO 7 / EN 10226, often marked R (external) and Rc (internal). Whitworth 55° form. Taper is 1:16 on diameter — numerically the same 3/4" per foot family — which is why a BSPT nipple will catch on NPT and convince you it “fits.” Pitch matches NPT on some sizes (1/2" and 3/4" are both 14 TPI) and diverges on others (1/4" BSP is 19 TPI; 1/4" NPT is 18).

There is no safe “close enough.” A mixed joint may hold shop air for a week and fail on oil. If the fitting is from a European valve or a imported compressor kit, assume BSP until you prove NPT with TPI + OD + a thread gauge.

Straight / Parallel Pipe Thread (BSPP and NPSM)

BSPP (G) is parallel 55°. It seals on a bonded washer (Dowty) or a face O-ring, not on the threads. NPSM is the US straight mechanical pipe thread (60°) used with a seat — common on some drop-ear elbows and free-spinning nuts.

If you can screw the parts together past where NPT would have bound, and the crests look uniform the whole length, you are probably holding parallel. Wrapping them with tape and jamming into a taper fitting is the leak in the troubleshooting guide.

Which One Should You Spec?

• US job, US fittings, no import valves: NPT (or NPTF if the spec is dryseal).
• Imported pump, ISO port marked G or R: stay in the BSP family. Do not “adapt” with brute force; use a marked adapter.
• Swivel nut onto a seat: parallel (NPSM or BSPP) plus the washer the manufacturer sold with it.
• Welding the other end of the same nipple: thread choice does not change weld procedure — use the pipe welding calculator for the bevel, and keep the thread standard consistent on the threaded end.

How to Identify the Thread in Your Hand

1. Count threads in a measured inch. Compare to the NPT TPI column on the NPT chart. 19 TPI on a small fitting is a BSP tell.
2. Measure OD. 1/2" NPT sits near 0.840". A caliper that reads closer to 0.825" with 14 TPI is a BSP suspect, not a worn NPT.
3. Look at the crest with a 60° vs 55° gauge if you have one. Eyes lie; the gauge does not.
4. See whether diameter changes along the thread. No change → parallel. Change → taper (then distinguish NPT vs BSPT with pitch/OD).

FAQ