A fitting that "almost" screws together is a fitting that will eventually leak. Thread standards that look similar can differ in flank angle, pitch or form, and brass — a relatively soft metal — will be damaged if they are forced together. These are the families we are asked for most often.
| Thread | Standard | Form | Typical use |
|---|---|---|---|
| BSPP (G) | ISO 228-1 | Parallel, 55° | Seals with a washer or O-ring; Europe, Asia, Middle East |
| BSPT (R / Rc) | ISO 7-1 | Taper, 55° | Seals on the thread with sealant |
| NPT | ASME B1.20.1 | Taper, 60° | North American pipe threads |
| Metric (M) | ISO 261 / 262 | Parallel, 60° | Sensors, glands, fasteners |
| UNC / UNF | ASME B1.1 | Parallel, 60° | Imperial fasteners and fittings |
Parallel vs taper
Parallel threads do not seal by themselves; a bonded washer, O-ring or flat face provides the seal. Taper threads wedge together and seal on the thread flanks, usually with PTFE tape or sealant. Specifying the wrong type is a design error, not a manufacturing one.
Do not mix BSPT and NPT
Some sizes share the same threads per inch, which makes the two look interchangeable. The flank angles differ (55° versus 60°), so the joint may start to engage but will not seal reliably and can damage both parts.
What to put on the drawing
- Thread standard and designation, e.g. "G 1/2 – ISO 228-1, class A"
- Internal or external thread, and the usable length
- Tolerance class or gauge requirement (go / no-go)
- Finish or plating that will affect thread dimensions
We gauge threads with plug and ring gauges during production and can supply a gauge record on request. Browse our fittings and inserts and fasteners for standard thread options.
Have a drawing or specification in mind? Send it to our export desk and we will respond with a clear quotation.