How to measure the thread size of an air line coupling

Thread sizes are based on trade sizes rather than actual dimensions, this guide shows how to measure a thread as well as identifying it as a British Standard Pipe (BSP).

There are two types of thread:

• Parallel (BSPP) - also known as G or Rp
• Tapered (BSPT) - also known as R or Rc

R, Rp and Rc threads are taken from BS21:1985 pipe threads for tubes and fittings where pressure-tight joints are made on the threads. G threads are taken from BS2779:1986 pipe threads for tubes and fittings where pressure-tight joints are usually made on a washer or an ‘O’ ring.

Measure across the outside diameter of a male or hose tailpiece thread or measure the inside diameter of a female thread.