Whether you’re a hobby plumber, devoted DIY-er or lover of industrial-style projects, the very first step is knowing how to cut, thread and assemble your galvanised pipe correctly. Follow our step-by-step tutorial to cut and thread steel pipe. Next, to understand the standards that govern these products, read the essential EN 10241 / ISO 7-1 guide to threaded galvanised pipe. Skipping these details quickly leads to leaks, costly incompatibilities and weak joints. This article zooms in on the threads used on Home Invasion products—malleable-iron fittings EN 10242 and steel pipes EN 10255—so you can get it right the first time.
1. What is a thread?
A thread is a helical ridge—flanks—that lets two parts screw together. To identify one, remember:
- Nominal size: stated in inches (¼", ½"…) and different from the actual outside diameter.
👉 Need a quick conversion? Check our DN-mm-inch diameter chart. - Pitch: distance between two threads, in TPI (threads per inch) or millimetres.
- Flank angle: 55 ° for BSP threads (ISO 7-1 or ISO 228-1) and 60 ° for NPT (ASME B1.20.1).
- Taper: parallel (constant diameter) or tapered 1 : 16 for BSPT / NPT.
2. The three main plumbing threads
2.1 BSPT – British Standard Pipe Taper (ISO 7-1 / EN 10226-1)
- 55 ° profile, slightly truncated crests for good bite.
- Taper 1 : 16 → mechanical seal by tightening plus PTFE tape or jointing compound.
- Uses: air, gas, low-pressure steam, industrial décor builds in galvanised pipe.
Male BSPT example
Female tapered thread
2.2 BSPP – British Standard Pipe Parallel (ISO 228-1)
- Parallel, still 55 °.
- Sealed with a flat washer, bonded seal or cone seat + O-ring.
- Uses: potable water, taps, hydraulics, instrumentation.
Female BSPP example
2.3 NPT – National Pipe Taper (ASME B1.20.1)
- Taper 1 : 16, 60 ° flanks, flat crests.
- Not compatible with BSPT / BSPP without an adapter.
- Common on North-American HVAC and pneumatic equipment.
Quick comparison
| Type | Shape | Angle | Sealing | Applications |
|---|---|---|---|---|
| BSPT | Taper 1 : 16 | 55 ° | PTFE / compound | Air, gas, industrial décor |
| BSPP | Parallel | 55 ° | Washer / cone 30 ° | Water, taps, hydraulics |
| NPT | Taper 1 : 16 | 60 ° | PTFE / compound | US equipment, industry |
3. Compatibility & adapters
| Male | Female | Compatibility | Sealing method |
|---|---|---|---|
| BSPT | BSPT | ✅ | PTFE or compound |
| BSPT | BSPP | ⚠️ Up to ≈10 bar | Thick PTFE – adapter safer |
| BSPP | BSPP | ✅ | Flat / O-ring washer |
| NPT | NPT | ✅ | PTFE or compound |
| NPT | BSPT / BSPP | ❌ | Adapter required |
You’ll find ready-made solutions in our black cast-iron adapter range.
4. Recommended maximum pressure (EN 10242 malleable iron, PN 25)
| Nominal size | Max pressure 20 °C |
|---|---|
| 1/8" – ½" | 25 bar |
| ¾" – 2" | 20 bar |
| 3" – 4" | 16 bar |
Need higher ratings? See the EN 10241 guide for heavy-duty classes.
5. Sealing techniques
5.1 Tapered threads (BSPT / NPT)
- PTFE tape: 3–5 clockwise wraps, one thread overhang.
- Compound + hemp: hot water, heating, gas (GASTEC approved).
- Tighten ≈1½ turns past finger-tight for PN 25 iron.
Parallel male thread
5.2 Parallel threads (BSPP)
- Flat fibre/EPDM washer – plumbing.
- Bonded steel/rubber washer – air & hydraulics.
- 30 ° cone seat + O-ring – high pressure.
6. Inch ↔ millimetre conversion
| Nominal inches | Schedule 40 OD (mm ± tol.) |
|---|---|
| 1/8" | 10 |
| 1/4" | 13 |
| 3/8" | 17 |
| 1/2" | 21 |
| 3/4" | 27 |
| 1" | 34 |
| 1 ¼" | 42 |
| 1 ½" | 48 |
| 2" | 60 |
| 3" | 89 |
| 4" | 114 |
7. Common mistakes to avoid
- Mixing BSPT, BSPP and NPT—identify before you buy. (All threads on our site are BSPT/BSPP and cross-compatible.)
- Fitting without cleaning the threads.
- Wrapping PTFE counter-clockwise.
- Forgetting the washer on BSPP.
- Over-tightening: can crack iron—use moderate torque.
- Skipping a pressure test after assembly.
8. Want to go deeper?
• Official steel-pipe sizes • How to cut & thread • EN 10241 explained
Conclusion
By mastering the differences between BSPT, BSPP and NPT threads—and the standards behind them—you’ll achieve tight, durable and safe assemblies for plumbing or industrial-style projects. Ready to build? Explore our pre-cut galvanised pipes to turn your ideas into professional-grade results.