Hydraulic fitting thread types: BSP, NPT, UNF (JIC), ORFS and metric
How to tell BSP, NPT, UNF (JIC), ORFS and metric DIN 2353 threads apart, how to measure them and why they must never be forced together.
"A 1/2 inch thread" is not enough in hydraulics to choose a fitting. Behind the same diameter hide several incompatible standards: BSP, NPT, UNF (JIC), ORFS and metric threads. A connection of the wrong parts may look like a fit, but it will not seal and, in the worst case, will leak under pressure. Below - how to tell them apart.
The main connection types
| Type | Thread | How it seals | Where you find it |
|---|---|---|---|
| BSP (G) | inch-based pipe thread, 55° profile, parallel | with a washer or an O-ring (the thread itself does not seal), or with a 60° cone on swivel connections | European hydraulics, pump and cylinder ports, water and pneumatic installations |
| BSPT (R, Rc) | inch-based pipe thread, 55°, tapered | on the thread, with tape or paste | piping installations |
| NPT | inch-based pipe thread, 60° profile, tapered | on the thread, with tape or paste | American equipment, piping installations |
| UNF / JIC | inch-based UN, 60° profile | 37° cone - metal to metal | industrial and machinery hydraulics |
| ORFS | inch-based UN | face seal, flat surface with an O-ring | high-pressure installations, where leak-tightness matters |
| Metric DIN 2353 | metric fine pitch, 24° cone | compression ring on the tube | European hydraulics |
How to identify a thread step by step
- Measure the outside diameter of the thread (for a male thread) with calipers. The result in millimetres approximates the size and tells you whether it is inch or metric.
- Check the pitch with a thread gauge (inch threads are given in threads per inch, metric in millimetres).
- Check the thread shape: parallel or tapered (narrowing towards the end).
- Check the sealing method: a cone (what angle?), a flat surface with an O-ring, or a plain washer.
Remember the look-alikes that are easy to confuse:
- JIC (37°) and SAE 45° (flare) look similar and have the same thread, but the cone angle is different - they are not interchangeable.
- NPT and BSPT are both tapered threads in similar sizes, but with a different profile angle (60° and 55°) and pitch - do not force them together.
- BSP and NPT in some sizes even screw together, yet they still do not seal.
Metric threads and the L and S series
In compression ring fittings (DIN 2353) the same metric thread can belong to two different tube series: light (L) and heavy (S). For example, an M14x1.5 thread fits an 8 mm tube in the L series and a 6 mm tube in the S series. That is why, besides the thread, you need to know the tube diameter and the series when ordering - in our shop this is a separate parameter, "Tube series" (L, S, LL).
How to choose a fitting
To choose the type and size you can use the wizard in the Nipples section (the "Configure" button): we ask about the shape, both ends (thread, size, gender), the material and, if needed, the tube series, and at the end we show the matching items with prices. If in doubt, send us a photo or the symbol of the old fitting.
This article is for information only. Thread dimensions and tolerances are defined by the relevant standards; for safety-critical applications follow the manufacturer's documentation.