BSP vs NPT Threads: Sealing Style and Identification
HGW Hydraulics on Jan 19th 2022
BSP and NPT are both pipe thread styles used to join fittings and pipes, but they are not interchangeable. They differ in thread angle, thread shape, common geography, and sealing method.
BSP is common in the United Kingdom and many Commonwealth markets. NPT is the dominant pipe thread style in North America. Both appear in hydraulic, industrial, oil and gas, power, manufacturing, chemical processing, and shipboard systems.
Thread Form Differences
BSP stands for British Standard Pipe. BSP threads have rounded peaks and valleys with a 55 degree thread angle.
NPT stands for National Pipe Taper. NPT threads have sharper peaks and valleys with a 60 degree thread angle. This difference is enough to make a near fit unreliable even when the size appears similar.
BSPP and BSPT
BSPP is the parallel style, sometimes marked as G thread. The diameter stays constant along the fitting. BSPP connections often seal with an O-ring, bonded seal, retaining ring, crush washer, or other sealing washer rather than by thread wedging alone.
BSPT is the tapered style, sometimes marked as R thread. The thread diameter changes along the length. A BSPT connection can seal by thread wedging, and sealant is often used to support a pressure-tight connection.
NPT and NPTF
NPT is a general-purpose tapered pipe thread. It normally requires thread sealant to create a leak-resistant joint. NPTF is similar in form but is designed to make a mechanical dryseal in selected applications.
NPT fittings are used in plumbing, gas conveyance, and some hydraulic systems, especially lower-pressure or general-purpose applications. They can leak in higher-pressure hydraulic work if the connection is over-tightened, reused improperly, or installed into a damaged female port.
Why They Should Not Be Forced Together
An NPT male thread can sometimes appear close to a BSPP female thread, but the pitch and thread form are different. Forcing the connection can misalign the threads, damage the port, and create a leak path under pressure.
Over-tightening is also risky. A tapered thread can crack a female port if it is tightened repeatedly or used to compensate for the wrong thread style.
How to Check Before Replacing
Identify whether the thread is parallel or tapered, measure the thread diameter, check threads per inch, inspect the seat or sealing washer location, and confirm whether the fitting is BSPP, BSPT, NPT, or NPTF.
Material selection still matters. BSP and NPT fittings may be made from brass, carbon steel, or stainless steel depending on pressure, media, corrosion exposure, and the system design.