The four systems at a glance
| System | Standard | Flank angle | Form | Seals on the thread? | Example callout |
|---|---|---|---|---|---|
| ISO metric | ISO 68-1, ISO 261, ISO 965 | 60° | Parallel | No | M8x1.25-6g |
| BSPP (parallel pipe) | ISO 228-1 | 55° | Parallel | No, seals on a washer or face | G 1/2 |
| BSPT (taper pipe) | ISO 7-1 / EN 10226 | 55° | Taper 1:16 | Yes, with sealant | R 1/2 (external), Rp / Rc 1/2 (internal) |
| NPT (taper pipe) | ASME B1.20.1 | 60° | Taper 1:16 | Yes, with sealant | 1/2-14 NPT |
| UNC / UNF (unified inch) | ASME B1.1 | 60° | Parallel | No | 1/4-20 UNC-2A |
ISO metric threads
The default fastening thread worldwide. It is called out by diameter and pitch in millimetres; if the pitch is omitted (M8), the coarse pitch is meant. Tolerance classes follow ISO 965: 6g for external threads and 6H for internal threads is the common medium fit.
| Size | M3 | M4 | M5 | M6 | M8 | M10 | M12 | M16 | M20 |
|---|---|---|---|---|---|---|---|---|---|
| Pitch (mm) | 0.5 | 0.7 | 0.8 | 1.0 | 1.25 | 1.5 | 1.75 | 2.0 | 2.5 |
Pipe threads: BSP and NPT
Pipe threads are named after a nominal pipe size, not the measured diameter, so a 1/2 inch pipe thread is roughly 21 mm across. Parallel threads (BSPP, marked G) seal on a washer, O-ring or face. Taper threads (BSPT and NPT) wedge tight and seal on the thread itself with tape or sealant.
| Nominal size | 1/8 | 1/4 | 3/8 | 1/2 | 3/4 | 1 | 1-1/4 to 2 |
|---|---|---|---|---|---|---|---|
| BSP (BSPP / BSPT) | 28 | 19 | 19 | 14 | 14 | 11 | 11 |
| NPT | 27 | 18 | 18 | 14 | 14 | 11.5 | 11.5 |
Unified inch threads: UNC and UNF
Used on equipment designed to inch standards, mainly for North America. The callout gives the diameter, threads per inch and series: 1/4-20 UNC is coarse, 1/4-28 UNF is fine. Classes 2A (external) and 2B (internal) are the usual general-purpose fit.
How to identify a thread on a sample
- 01Measure the outside diameter of the external thread with a caliper.
- 02Check whether the diameter changes along the length. If it does, the thread is tapered (BSPT or NPT).
- 03Measure the pitch with a thread pitch gauge in both millimetres and threads per inch, and see which fits cleanly.
- 04Compare the flank angle: 55° points to BSP, 60° to metric, NPT or unified.
- 05Match diameter and pitch against the tables above, then confirm with a go / no-go gauge of that standard.
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