How the thread identifier works
Process connections on pressure transmitters, gauges, temperature sensors and thermowells use a handful of thread families. Many of them look alike, and some share the same pitch, so the reliable way to identify a thread is to measure two things and compare them with the standard dimensions:
- The outside diameter of a male thread, across the crests
- The pitch, as threads per inch (TPI) or the distance between crests in millimetres
The identifier compares your measurement with the reference table and lists the best match and up to two alternatives. A thread matches when the pitch is within 4 % and the diameter is within 0.6 mm or 4 %, whichever is larger. Candidates are ranked by their combined deviation.
The thread families
| Family | Standard | Form | Flank angle | Seals on |
|---|---|---|---|---|
| G (BSPP) | ISO 228-1 | Parallel | 55° | Gasket, bonded seal or O-ring |
| R (BSPT) | ISO 7-1 | Taper 1:16 | 55° | The thread, with sealant |
| NPT | ASME B1.20.1 | Taper 1:16 | 60° | The thread, with sealant |
| Metric M | ISO 261 | Parallel | 60° | Sealing ring or gland |
| UNF (O-ring boss) | SAE J1926-1 | Parallel | 60° | O-ring in a chamfer |
| PG | DIN 40430 | Parallel | 80° | Cable gland seal |
Example: G 1/4 or 1/4 NPT?
A male thread measures 13.1 mm across the crests and has 19 threads per inch. The identifier returns G 1/4 (13.157 mm, 19 TPI) as the best match and R 1/4 as the alternative. G and R share the same dimensions; G is parallel and R is tapered. To tell them apart, check whether the diameter shrinks toward the end: a taper thread loses about 1 mm of diameter over 16 mm of length.
1/4 NPT is close in diameter (13.716 mm) but has 18 TPI and a 60° flank. Over an inch the difference is one full thread, enough to cross-thread and leak.
Another common confusion: G 1/2 and 1/2 NPT both have 14 threads per inch. G 1/2 measures 20.955 mm, 1/2 NPT about 21.3 mm, and the flank angles differ. They may screw together a few turns, but they won’t seal.
How to measure accurately
- Diameter: use calipers across the crests. On a tapered thread, measure two or three threads in from the end, where the thread is at full profile.
- Pitch: a thread gauge is best. Without one, count the crests over a measured length: threads over 25.4 mm gives TPI directly. Count over the longest length you can for accuracy.
- Female threads: measure the inside diameter across the crests and add roughly one thread depth, or better, measure the mating male part.
If you enter the diameter in inches or the pitch in millimetres, select the unit; the value converts automatically.
Why “1/2 inch” is not half an inch
G, R and NPT sizes come from historical pipe sizes, roughly the bore of the pipe, not the thread diameter. A 1/2” pipe thread measures about 21 mm (0.84 in) across the crests. This is the most common reason threads are misidentified by a simple measurement.
The reference table
The table lists all threads with major diameter, TPI, pitch, flank angle and how each one seals. Filter it by process connections or cable entries; matches are highlighted, and clicking a row loads its nominal size into the measurement fields. For taper threads, the diameter listed is the nominal pipe outside diameter used for identification.
Common mistakes
Using PTFE tape on parallel threads. G and metric threads seal on a gasket or O-ring, not on the thread. Tape can stop the sealing face from closing.
Mixing NPT and R. They have the same taper but different flank angles and, except for 1/2” and 3/4”, different pitches. Even where pitches match, the joint is unreliable.
Over-tightening tapered threads into soft housings. Taper threads wedge; excessive torque can crack aluminium or plastic housings.
Frequently asked questions
Which thread do pressure transmitters use?
In Europe G 1/2 and G 1/4 are most common, often with a sealing ring; in North America 1/2 NPT and 1/4 NPT. Hydraulic equipment often uses SAE O-ring boss threads (7/16-20, 9/16-18) or metric threads.
What does “-4” or “-6” mean on SAE fittings?
The dash size gives the tube outside diameter in sixteenths of an inch: -4 is 1/4” tube with a 7/16-20 UNF thread, -6 is 3/8” tube with a 9/16-18 UNF thread.
Are PG threads still used?
PG threads (DIN 40430) appear on older equipment and cable glands; new designs mostly use metric M threads (M16, M20, M25 × 1.5) for cable entries.
Can I share the result?
Yes. “Copy link” creates a URL with your measurement already entered.