The BSP (British Standard Pipe) thread is a family of standard screw thread types that has been adopted internationally and are now denoted by R (external taper), Rp (internal parallel), Rc (internal taper) & G (external & internal parallel).
R, Rp, Rc & G thread sizes
A list of thread sizes as defined by BS 21:1985 and BS 2779:1986 standards are detailed below.
The "Thread size" was originally based on the inner diameter, in inches (i.e. 1/4"), of the tube for which the thread was intended. Now, in the modern metric version, it is simply a size number.
Thread
size |
Threads
per inch |
Pitch |
Major diameter OD |
Gauge
length |
| |
TPI
|
mm
|
mm
|
inch
|
mm
|
| 1/16 |
28 |
0.907 |
7.723 |
0.304 |
4 |
| 1/8 |
28 |
0.907 |
9.728 |
0.383 |
4 |
| 1/4 |
19 |
1.337 |
13.157 |
0.518 |
6 |
| 3/8 |
19 |
1.337 |
16.662 |
0.656 |
6.4 |
| 1/2 |
14 |
1.814 |
20.995 |
0.825 |
8.2 |
| 3/4 |
14 |
1.814 |
26.441 |
1.041 |
9.5 |
| 1 |
11 |
2.309 |
33.249 |
1.309 |
10.4 |
The "Major diameter OD" is the outer diameter of the external thread.
For a taper thread - it is the diameter at the the "Gauge length" from the narrowest (most tapered) end of the thread. The taper is 1:16, which means that for each 16 units of measurement increase in the distance from the end, the diameter increases by 1 unit of measurement.
For a parallel thread - it is constant.