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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 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.
| 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 |
