Comparison
BSP vs NPT
BSP (British, 55° Whitworth form) and NPT (American, 60°) are incompatible pipe-thread families that measure deceptively close. BSPP is parallel and seals on a cone or washer; NPT/BSPT are tapered and seal on the threads. Mixing families produces the classic slow leak that no torque cures.
Side by side
The decision table.
| BSP | NPT | |
|---|---|---|
| Flank angle | 55° | 60° |
| Forms | BSPP parallel / BSPT taper | NPT/NPTF taper |
| Seal | Cone seat, washer, or O-ring (BSPP) | Thread interference |
| 3/8" pitch | 19 TPI | 18 TPI |
| Common on | UK/EU/Japanese equipment | US equipment and ports |
Choose BSP when…
- Imported equipment with G/R-marked ports and unions
Choose NPT when…
- Domestic ports, gauges, and legacy US plumbing
What happens if you mix them
NPT and BSPT will start into each other at several sizes and even hold briefly. Heat cycles open the mismatched flanks and the joint weeps forever — identify with pitch and angle gauges before wrenching.
BSP vs NPT Threads — FAQs
Why does my imported machine leak after US-sourced fittings?
Almost always NPT parts in BSP ports. We stock the correct BSPP/BSPT fittings and BSP-to-NPT conversion adapters.
Is Teflon tape the fix for a BSP/NPT mismatch?
No. Sealant can't bridge a flank-angle mismatch under hydraulic pressure. Fit the correct thread.
Still deciding?
Bring both fittings to the counter — or call (817) 589-7101 and read us the layline. We settle these in minutes, with the gauges out.