How to Identify Pipe Thread Types

What it is

Pipe threads come in two main families: American (NPT and its relatives, 60° thread angle) and British Standard Pipe (BSP, 55° thread angle). Each family has a tapered version that seals on the threads and a parallel (straight) version that needs a gasket, O-ring or bonded seal. Some sizes in the two families look almost the same, which is why they get mixed up.

• NPT: 60°, tapered; seals by threads plus sealant or tape (ASME B1.20.1).
• NPTF (Dryseal): 60°, tapered; seals by thread crest and root contact; sealant optional (ASME B1.20.3).
• NPSM / NPS: 60°, straight; seals by gasket, or a seat on a mechanical joint (ASME B1.20.1).
• BSPT (R / Rc): 55°, tapered; seals by threads plus sealant (ISO 7-1).
• BSPP (G): 55°, parallel; seals by bonded washer, O-ring or gasket (ISO 228-1).

How to identify

1. Check the taper. Lay a straightedge along a male thread or look down its profile. Tapered threads get visibly smaller toward the end. Parallel threads stay the same diameter. A female tapered thread lets a male thread in only a short distance by hand.
2. Measure the OD of the male thread with a caliper and compare it to the table.
3. Count the threads per inch with a pitch gauge. Try the inch (TPI) and the Whitworth/BSP leaves of the gauge. In sizes where the pitch is the same, use the OD.
4. Check the thread angle (60° vs 55°) with a thread-angle gauge if the count is still unclear.
5. Look for markings. Imported fittings are often stamped "R", "Rc", "G" or "BSP".

Threads per inch by size

Key points

• 1/2 and 3/4 NPT and BSP threads have the same pitch (14 TPI), so they can start together. The joint will not seal reliably.
• Tapered threads are measured at the small end. The OD over the first few threads is a bit smaller than the pipe OD.
• NPTF is made for sealing without sealant, but it mates with NPT. A mixed NPT/NPTF joint should be treated as NPT and sealed.
• Garden hose (GHT) and fire hose threads are separate thread forms and are not pipe threads.

Common mistakes

• Forcing a BSPT fitting into an NPT port, or the reverse.
• Using thread tape on a BSPP or NPSM joint that seals with a gasket or O-ring.
• Identifying a thread by pitch alone in the 1/2 and 3/4 sizes.

Governing standards: ASME B1.20.1 (NPT and NPS); ASME B1.20.3 (Dryseal NPTF); ISO 7-1 (BSPT, R and Rc); ISO 228-1 (BSPP, G).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.