BSP (British Standard Pipe) is the most commonly used pipe thread standard in the UK. There are two types: BSPP (parallel) which seals on a washer or O-ring, and BSPT (tapered) which seals on the thread itself using PTFE tape or thread sealant.
In a hurry? These are the things you need to know:
-
To measure a BSP thread, measure the outer diameter (OD) of a male thread or the inner diameter (ID) of a female thread using a vernier calliper. BSP thread sizes are named by nominal bore, the actual measured diameter is always larger than the nominal size.
-
Check the thread pitch using a pitch gauge. Common BSP pitches include 19 TPI (threads per inch) for 1/2" BSP and 14 TPI for 3/4" and 1" BSP.
-
Parallel (BSPP) and tapered (BSPT) BSP threads have the same pitch and nominal diameter, making them easy to confuse. Identifying which type you have determines the correct sealing method.
-
If in doubt, use a thread identification kit, these include a range of gauges that test both diameter and pitch simultaneously, making positive identification straightforward without specialist knowledge.
Identifying BSP Threads
One of the most frequent problems that we have found our customers run into is the identification and measurement of pipe threads.
In Great Britain BSP British Standard Pipe threads are commonly used. These are either parallel (BSPP) which normally seal on a washer or 'O' ring seal, or tapered (BSPT) which taper along their length and normally seal by two mating threads.
How to measure your thread:
Use a tape rule (or vernier gauge) to measure across the outside of the male thread. Note that a thread that measures 1/2" is not a 1/2" BSP thread.
Using the table below we can see that a thread measuring 1/2" is actually a 1/4" BSP thread. Similarly, a thread that measures 1" is actually a 3/4" BSP thread.
| BSP Size | Max. diameter | Min. diameter | Thread pitch | ||
|---|---|---|---|---|---|
| inch | mm | inch | mm | inch | (mm) TPI |
| 1/16" | 7.723 | 0.304 | 6.561 | 0.258 | 28 |
| 1/8" | 9.728 | 0.383 | 8.565 | 0.337 | 28 |
| 1/4" | 13.157 | 0.518 | 11.445 | 0.451 | 19 |
| 3/8" | 16.662 | 0.656 | 14.950 | 0.589 | 19 |
| 1/2" | 20.955 | 0.825 | 18.633 | 0.734 | 14 |
| 5/8" | 22.911 | 0.902 | 20.589 | 0.811 | 14 |
| 3/4" | 26.441 | 1.041 | 24.120 | 0.950 | 14 |
| 1" | 33.249 | 1.309 | 30.292 | 1.193 | 11 |
| 1.1/4" | 41.910 | 1.650 | 38.953 | 1.534 | 11 |
| 1.1/2" | 47.803 | 1.882 | 44.846 | 1.766 | 11 |
| 2" | 59.614 | 2.347 | 56.657 | 2.231 | 11 |
| 2.1/2" | 75.184 | 2.960 | 72.227 | 2.844 | 11 |
| 3" | 87.884 | 3.460 | 84.927 | 3.344 | 11 |
| 4" | 113.030 | 4.450 | 110.073 | 4.334 | 11 |
| 5" | 138.430 | 5.450 | 135.472 | 5.334 | 11 |
| 6" | 163.830 | 6.450 | 160.872 | 6.334 | 11 |
Other Threads
In Europe Metric (DIN) pipe threads are common on German machines. These generally have a 1.5mm thread pitch, although this can vary. Smaller sizes can be 1.0mm thread pitch, and larger sizes can be 2.0mm thread pitch. Other variations occur. French equipment often have GAZ fine threads with a 24 degree cone.
In North America NPT, JIC and ORFS pipe threads are common. Some NPT threads are similar to BSPT threads so if in doubt check origin of part.
In Japan JIS tapered and parallel threads are common and are interchangeable with the respective BSPT and BSPP threads, although the 30 degree cones might be inverted.
