We get a lot of questions from our customers regarding thread type on everything from fire hose to fire hose fittings and adaptors. In the fire protection industry there are two main thread types - fire hose thread and pipe thread.
Fire hose thread (or simply 'hose thread') is typically expressed as NST (or NH) thread. It is a straight, coarse thread most commonly used on fire hose. People often think they need 2" or 3" NST thread based on the measurements of their existing parts or equipment. But more often than not, they actually need 1½" or 2½" NST thread. Just for reference the outside diameter (OD) of 1½" NST thread measures 1.99" with 9 threads per inch. The OD of 2½" NST thread measures 3.0686" with 7½ threads per inch.
Pipe thread is an entirely different matter. It is available in both straight and tapered varieties, both with fine, as opposed to coarse threads. The straight type can be expressed as NPSH (or IPS) thread , while the tapered type is expressed as NPT (or IPT) thread. Although it's called pipe thread, fire hoses are commonly available with pipe thread couplings, usually the straight NPSH type. The OD of 1½" NPSH thread measures 1.879" with 11½ threads per inch, while the 2½" NPSH thread measures 2.846" with 8 threads per inch. For extra confusion please note, a male NPT thread will fit a female NPSH thread, but a male NPSH thread will NOT fit a female NPT thread.
if you have any questions, please do not hesitate to contact us.