⚡ Key Takeaways
- Metric M: 60° thread form, dimensions in millimetres; the norm in China, Europe and Japan.
- Unified inch UNC/UNF: 60° thread form, inch size plus threads per inch (TPI); the norm for US equipment.
- Pipe threads G/R/NPT: 55° or 60°, used for sealing fluid connections.
- The key point: M6×1.0 and 1/4-20 UNC are not interchangeable — the pitches differ.
- Standards: GB/T 196, GB/T 197, GB/T 20669-2006 and ASME B1.1.
📑 Contents
1. The three thread families
2. Common cross-reference
3. Pipe threads: G versus NPT
4. Pipe thread drill sizes
5. Property classes versus SAE grades
6. Selection FAQ
1. The three thread families
2. Common cross-reference
3. Pipe threads: G versus NPT
4. Pipe thread drill sizes
5. Property classes versus SAE grades
6. Selection FAQ
1. The three thread families
| Family | Thread angle | Designation | Standard | Application |
|---|---|---|---|---|
| Metric M | 60° | M8×1.25 | GB/T 196, ISO 724 | General fastening |
| Unified UNC/UNF | 60° | 1/4-20 UNC | ASME B1.1, GB/T 20669 | US equipment |
| Whitworth pipe G/R | 55° | G1/2 | ISO 228, GB/T 7307 | Water and air lines |
| NPT pipe | 60° taper | 1/2-14 NPT | ASME B1.20.1 | Hydraulics, US pipework |
| German PG | 80° | PG11 | DIN 46271 | Cable glands |
2. Common cross-reference
| Metric | Inch UNC/UNF | Approximate hole size (mm) |
|---|---|---|
| M3×0.5 | No.5-40 UNC | 2.5 |
| M4×0.7 | No.8-32 UNC | 3.3 |
| M5×0.8 | No.10-24 UNC / No.10-32 UNF | 4.2 |
| M6×1.0 | 1/4-20 UNC / 1/4-28 UNF | 5.2 |
| M8×1.25 | 5/16-18 UNC | 6.8 |
| M10×1.5 | 3/8-16 UNC | 8.5 |
| M12×1.75 | 1/2-13 UNC | 10.2 |
Important: the table gives approximate hole sizes, not interchangeable threads. M6 and 1/4-20 have different pitches (1.0 versus 1.27 mm), so forcing one into the other damages the thread.
3. Pipe threads: G versus NPT
- G (parallel): 55°, does not seal on the thread — needs an O-ring or gasket; used in Europe and China;
- R (tapered): 55° taper 1:16, seals on the thread; used in Europe;
- NPT: 60° taper 1:16, sealed with PTFE tape; the US standard;
- G 1/2 and NPT 1/2 are not interchangeable — the thread angle differs.
4. Pipe thread drill sizes
| Pipe thread | TPI | Drill size (mm) | Use |
|---|---|---|---|
| G1/8 | 28 | 8.8 | Instruments, pneumatics |
| G1/4 | 19 | 11.8 | Pneumatics, small hydraulics |
| G3/8 | 19 | 15.2 | Hydraulics, oil lines |
| G1/2 | 14 | 19.0 | Water, medium hydraulics |
| G3/4 | 14 | 24.5 | Water lines |
| G1 | 11 | 30.7 | High flow |
5. Metric property classes versus SAE grades
| Metric class | Tensile (MPa) | SAE grade | Hardness HRC |
|---|---|---|---|
| 4.6 | 400 | Grade 2 | ~HRB |
| 5.8 | 500 | Grade 3 | ~HRB |
| 8.8 | 800 | Grade 5 | 25–34 |
| 10.9 | 1040 | Grade 8 | 33–39 |
| 12.9 | 1220 | — (ASTM A574) | 39–44 |
Important: Grade 8 is not class 12.9 — Grade 8 corresponds to class 10.9. Do not substitute by matching the number when buying for US equipment.
6. Selection FAQ
Q: The machine is US standard — will an M bolt do? No. It will strip the thread.
Q: UNC versus UNF? UNC is coarse (vibration tolerant, easy to assemble), UNF is fine (self-locking, more precise).
Q: What drill for G1/2? About 18.6 mm for the tapping hole.
🛠️ Engineering support: we supply the full M / UNC / UNF / G / NPT range.
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