How to measure the thread size of an air line coupling
How to measure the thread size of an air line coupling
Thread sizes are based on trade sizes rather than actual dimensions. This guide explains how to measure a thread and identify 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 defined in BS21:1985 for pipe threads used in tubes and fittings where pressure-tight joints are made on the threads. G threads are defined in BS2779:1986 for pipe threads where pressure-tight joints are typically made on a washer or an ‘O’ ring.
Measure the outside diameter of a male or hose tailpiece thread, or measure the inside diameter of a female thread.
| Trade Size (BSP) | Threads Per Inch | Max Diameter | Min Diameter | ||
| Inch | mm | Inch | mm | ||
| 1/8 | 28 | 0.383 | 9.728 | 0.3372 | 8.656 |
| 1/4 | 19 | 0.518 | 13.157 | 0.4506 | 11.445 |
| 3/8 | 19 | 0.656 | 16.662 | 0.5886 | 14.95 |
| 1/2 | 14 | 0.825 | 20.955 | 0.7336 | 18.633 |
| 3/4 | 14 | 1.041 | 26.441 | 0.9496 | 24.12 |
| 1 | 11 | 1.309 | 32.249 | 1.1926 | 30.292 |
