Coarse (standard pitch)
Default for general assembly and usually more forgiving in manufacturing.
- If pitch is omitted (ex: M10), coarse is assumed.
- More robust against dirt and handling damage.
- Use locking features in vibration-heavy applications.
Standards
Metric threads: practical standards notes
Coarse/fine selection guidance, quick tap drill calculator, and compact workshop tables.
Fine (smaller pitch)
Provides more threads per length at the same diameter for finer adjustment.
- Works well in thin-wall sections.
- Improves clamp adjustment sensitivity.
- More sensitive to contamination and burrs than coarse threads.
Common pitch snapshot
Practical coarse/fine view from M3 to M24.
| Size | Coarse P (mm) | Fine P (mm) |
|---|---|---|
| M3 | 0.5 | 0.35 |
| M4 | 0.7 | 0.5 |
| M5 | 0.8 | 0.5 |
| M6 | 1 | 0.75 |
| M8 | 1.25 | 1 |
| M10 | 1.5 | 1.25 / 1 |
| M12 | 1.75 | 1.5 / 1.25 |
| M16 | 2 | 1.5 |
| M20 | 2.5 | 2 / 1.5 |
| M24 | 3 | 2 |
Mini Tool
Tap drill calculator
Approximate drill diameter: D - P x factor. The factor is adjusted by selected thread engagement.
Recommended drill diameter
8.75 mm
Round to the nearest practical drill size in your workshop.
Starting point for hard materials: 8.9 mm
Copy-ready callout
M10x1.5 - 6H/6g
Formula note
D_tap ≈ D_nominal - P x (engagement/77)
For harder materials, opening +0.10 / +0.20 mm often lowers tap load.
Clearance hole quick table
Close / normal / loose values (typical practical ranges).
| Size | Close | Normal | Loose |
|---|---|---|---|
| M3 | 3.2 | 3.4 | 3.6 |
| M4 | 4.3 | 4.5 | 4.8 |
| M5 | 5.3 | 5.5 | 5.8 |
| M6 | 6.4 | 6.6 | 7 |
| M8 | 8.4 | 9 | 10 |
| M10 | 10.5 | 11 | 12 |
| M12 | 13 | 13.5 | 14.5 |
| M16 | 17 | 18 | 19 |
| M20 | 21 | 22 | 24 |
Thread engagement length (rule of thumb)
Good for early design loops. Validate by standard/tolerance and critical-load checks.
| Material | Guideline | Note |
|---|---|---|
| Steel base material | 1.0 x D | Typical starting point for general machine design. |
| Cast iron (EN-GJL / EN-GJS) | 1.2 x D | Slightly longer engagement helps brittle base materials. |
| Aluminum alloy | 1.5 x D | Longer thread reduces stripping risk in softer alloys. |
| Brass / Bronze | 1.2-1.5 x D | Use upper range for frequent assembly/disassembly. |
| Plastics (PA6, POM, etc.) | 2.0 x D + insert | Prefer metal threaded inserts for durability. |
Fit and tolerance notes
- Common combination: internal 6H with external 6g.
- Add lead-in chamfers where threads are near locating features.
- In corrosive environments, use stainless fasteners with suitable anti-seize.
References
- ISO 68-1: Basic profile for ISO metric screw threads
- ISO 261: ISO general purpose metric screw threads - General plan
- ISO 965-1: Tolerances for metric screw threads
- Machinery's Handbook, latest edition (threading chapters)