How to measure the thread size of an air line coupling

When measuring thread size, there's often confusion as thread sizes are based on trade size rather than actual dimension. Here is our simple guide to measuring a thread and identifying it as a British Standard Pipe (BSP).

The BSP is the UK’s most commonly used type of thread and falls into two categories:

• 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. Meanwhile G threads are taken from BS2779:1986 pipe threads for tubes and fittings where pressure-tight joints are usually made on a washer or ‘O’ ring.

Using a ruler or vernier, measure across the outside diameter of a male or hose tailpiece thread or measure the inside diameter of a female thread.

If the thread measures 0.5" it is not a 1/2" BSP thread. Using the table below you can see that a measurement of 0.5" is actually a 1/4" BSP thread and that a 0.75" measurement is a 1/2" BSP thread. So, you can basically take 0.25".

Our catalogue also features an ‘actual size threadform profile’ chart on page 19, this enables coupling and adaptors with male threads to be matched to the outlines of those shown on the page to help you easily find the thread size.

Request or download a copy of PCL's catalogue here

Watch the full demonstration