BSP is the British pipe thread, NPT is the American pipe thread and metric threads are the ISO general-purpose threads. Indian brass suppliers cut all three every day, but they are not interchangeable, so your drawing must name the exact standard.
A wrong thread is one of the most common reasons brass fittings are rejected. Two threads can look identical by eye and still leak or jam.
The main standards
- BSP parallel, marked G, follows ISO 228-1. It does not seal on the thread and needs a washer, O-ring or gasket.
- BSP taper, marked R for external and Rc or Rp for internal, follows ISO 7-1 and seals on the thread with a sealant.
- NPT follows ASME B1.20.1. It is tapered and seals on the thread with sealant or tape.
- NPTF is a dry-seal version of NPT under ASME B1.20.3, used in some fuel and hydraulic parts.
- Metric threads follow ISO 261 and ISO 965, such as M6 x 1, with tolerance classes like 6g and 6H.
- Unified threads, UNC and UNF, follow ASME B1.1 and are common in North American hardware.
Why BSP and NPT do not mix
BSP threads have a 55-degree flank angle. NPT threads have a 60-degree angle. Several sizes also have different pitches. A BSP fitting may turn a few times into an NPT port and then bind or leak under pressure.
Sell the thread that your market uses. Most of Europe, the UK, the Middle East, India and Australia use BSP for plumbing. The US and Canada use NPT.
How to specify and check threads
Write the full thread designation on the drawing, such as G 1/2, R 3/4 or 1/2-14 NPT, with the class where the standard uses one. Do not write "1/2 inch thread".
Threads should be checked with go and no-go thread gauges for parallel threads, and with taper plug and ring gauges for BSP taper and NPT. Ask the supplier to show you its gauges and their calibration certificates.
Thread quality also includes the start and the finish. Burrs, torn threads and a sharp first thread can damage seals. Ask for a chamfer or a removed incomplete thread where it matters.
Mistakes to avoid
- Do not accept parts checked only with a mating fitting instead of a gauge.
- Do not mix part numbers for BSP and NPT versions in the same bag or carton.
- Do not forget that plating adds thickness and can make a tight thread fail the gauge.
Next step
Thread errors are cheap to prevent and expensive to find after delivery. Read how to write a specification for a turned brass component for the rest of the drawing. To set up gauge checks with your supplier, book a free sourcing review.