Threads — sizes, pitch & design rules
Quick reference for metric ISO threads: standard sizes, pitch, tap-drill and clearance-hole diameters, plus rules of thumb for minimum thread engagement and material around tapped holes.
What to consider
Minimum engagement
As a rule of thumb the thread length should be at least 1 × D in steel, 1.5 × D in aluminium and 2 × D in magnesium or plastic, where D is the nominal thread diameter.
Minimum wall / boss
Keep at least 1 × D of material around a tapped hole (boss diameter ≥ 2 × D, edge distance ≥ 1.5 × D). Going thinner risks blow-out and thread pull-out under load.
Minimum material below a blind hole
Leave at least 0.5 × D of unthreaded material below a blind tapped hole, and drill 2–3 pitches deeper than the required thread depth so the tap can cut full threads.
Coarse vs fine pitch
Use coarse as the default — more tolerant of damage, faster to assemble, better in soft materials. Use fine for thin walls, high preload, vibration resistance or precise adjustment.
Tolerance class
ISO 965 default is 6H / 6g (nut / bolt) — equivalent to a general fit. Use 4H/4h for precision, 7H/8g where coatings or hot-dip galvanising are applied.
Use inserts in soft material
In aluminium, magnesium or plastics that see repeated assembly, use steel thread inserts (Helicoil, Time-Sert, Ensat). The insert lets you keep the boss small while getting steel-on-steel thread strength.
Standard sizes (coarse pitch)
Pitch and tap-drill per ISO 261 / 724. Clearance hole is the medium series per ISO 273. Minimum engagement is a practical rule of thumb for class 8.8 bolts.
| Size | Pitch (mm) | Tap drill ⌀ (mm) | Clearance hole ⌀ (mm) | Min. engagement — steel | Min. engagement — alu |
|---|---|---|---|---|---|
| M3 | 0.5 | 2.5 | 3.4 | 3 mm | 5 mm |
| M4 | 0.7 | 3.3 | 4.5 | 4 mm | 6 mm |
| M5 | 0.8 | 4.2 | 5.5 | 5 mm | 8 mm |
| M6 | 1.0 | 5.0 | 6.6 | 6 mm | 9 mm |
| M8 | 1.25 | 6.8 | 9.0 | 8 mm | 12 mm |
| M10 | 1.5 | 8.5 | 11.0 | 10 mm | 15 mm |
| M12 | 1.75 | 10.2 | 13.5 | 12 mm | 18 mm |
| M14 | 2.0 | 12.0 | 15.5 | 14 mm | 21 mm |
| M16 | 2.0 | 14.0 | 17.5 | 16 mm | 24 mm |
| M20 | 2.5 | 17.5 | 22.0 | 20 mm | 30 mm |
| M24 | 3.0 | 21.0 | 26.0 | 24 mm | 36 mm |
| M30 | 3.5 | 26.5 | 33.0 | 30 mm | 45 mm |
Common fine pitches
Fine threads have a shallower helix — higher preload at the same torque, better hold in thin walls, but more sensitive to damage and contamination.
| Size | Coarse pitch | Fine pitch(es) | Typical use |
|---|---|---|---|
| M8 | 1.25 | 1.0 | Vibration-prone joints, thin walls |
| M10 | 1.5 | 1.25 / 1.0 | Hydraulics, adjusters |
| M12 | 1.75 | 1.5 / 1.25 | Precision fasteners |
| M16 | 2.0 | 1.5 | Hydraulic fittings |
| M20 | 2.5 | 1.5 | High preload, fine adjustment |
Common mistakes
- Too little engagement. A bolt that bottoms out before reaching full engagement will strip the threads, not break the bolt.
- Tapping right next to an edge. Plan for at least 1.5 × D edge distance — otherwise the wall bulges and the thread loses preload.
- Wrong drill size. Using the clearance hole as the tap drill is the classic mistake — the thread will only cut at the crest and strip on first tightening.
- Coarse thread in thin sheet. Below ~1.5 × pitch of sheet thickness you won't get a full thread — use a fine pitch, a nut, a rivnut or a weld nut instead.
- Forgetting the chamfer. A 90° lead-in chamfer (≈ 1 × pitch deep) prevents the first thread from rolling out and makes assembly much easier.