Metric fasteners are specified as diameter × pitch × length — for example M12 × 1.75 × 50. Pitch determines threads per millimetre; length determines grip and engagement. Coarse pitch is the default for most industrial carbon steel assembly; fine pitch appears where adjustment, shorter thread depth, or vibration resistance matters. Wrong pitch prevents nut fit; wrong length causes insufficient engagement or dangerous protrusion into moving parts.
Coarse vs fine pitch
Coarse pitch (standard) is the default for each metric diameter — e.g. M10 × 1.5, M12 × 1.75, M16 × 2.0. Fewer threads per mm means faster assembly and greater tolerance for slightly damaged threads in field maintenance.
Fine pitch uses smaller pitch value for same diameter — e.g. M10 × 1.25, M12 × 1.5. More threads per mm increases thread contact area and can improve resistance to loosening in vibration when properly torqued.
Bolt and nut pitch must match exactly. An M12 × 1.75 bolt will not start into an M12 × 1.5 nut — feels like cross-thread immediately.
| Diameter | Coarse pitch | Common fine pitch |
|---|---|---|
| M6 | 1.0 | 0.75 |
| M8 | 1.25 | 1.0 |
| M10 | 1.5 | 1.25 |
| M12 | 1.75 | 1.5 |
| M16 | 2.0 | 1.5 |
| M20 | 2.5 | 1.5 |
When to specify fine pitch
Fine pitch suits thin-wall tapped holes where coarse thread depth would weaken the parent material — aluminium plates, sheet metal brackets, and plastic inserts (with appropriate insert design).
Adjustable machinery stops, precision slide mounts, and some hydraulic flanges specify fine pitch for finer adjustment per turn.
Stock coarse pitch unless drawing explicitly calls fine — fine pitch is less common in UAE distributor stock and may extend lead time.
- Drawing calls M10×1.25 — order fine, do not substitute 1.5
- Helicoil and thread repair kits are pitch-specific
- Fine pitch more sensitive to contamination in dusty plants — clean threads before assembly
- Stainless fine pitch more prone to galling — use anti-seize
How bolt length is measured
Hex head bolts (DIN 933 / ISO 4017): length from underside of head to tip of shank. Partial thread bolts (DIN 931 / ISO 4014): same length measure; unthreaded grip length is longer portion under head.
Socket head cap screws (DIN 912): length typically overall from head bearing to tip unless drawing states otherwise — confirm on critical assemblies.
Countersunk screws: length often measured overall or to shoulder per standard — mismatched convention causes head proud or buried too deep.
Grip length and threaded portion
Grip length is the total thickness of materials clamped — plates, flanges, washers (excluding spring lock washer compression). Unthreaded shank should span the grip so threads engage only in nut or tapped hole, not across the joint interface.
Thread engagement in nut or tapped hole should be at least one times nominal diameter for steel (M12 bolt → minimum ~12 mm thread engagement in steel nut or hole). Soft materials (aluminium) need more — often 1.5–2× diameter or helicoil.
Excessive protrusion past nut wastes length, catches clothing, and may violate guard clearances — select next shorter standard length or trim only if spec allows.
| Joint type | Minimum engagement guidance |
|---|---|
| Steel nut on through bolt | ≥ 1× diameter thread engagement |
| Tapped steel hole | ≥ 1× diameter; 1.25× for dynamic load |
| Aluminium tapped | ≥ 1.5–2× diameter or insert |
| Thin sheet + nut | Use flange nut or washer; avoid single thread engagement |
Selecting standard lengths
Bolts are stocked in 5 mm increments on smaller sizes (M6–M16) and 10 mm on larger — choose nearest standard length meeting grip plus engagement without excessive stick-out.
Structural assemblies using partial thread bolts: ensure unthreaded shank spans all plies in grip — threads in shear plane reduce joint slip resistance under some design codes.
When replacing unknown bolt, measure grip stack with calipers and add engagement length for nut side — do not measure old bolt overall if tip is damaged or was wrong length originally.
Partial vs full thread selection
Partial thread (DIN 931 / ISO 4014) places smooth shank in bearing against clearance holes — preferred for structural steel connections and precise shear transfer.
Full thread (DIN 933 / ISO 4017) suits general through-bolting where entire length may need threads for varying stack thickness or where shear plane thread contact is acceptable.
Ordering partial thread requires minimum thread length on shank — very short bolts may be full thread by manufacturing necessity; check catalog for break point on diameter.
Tapped holes and thread repair
Drill and tap sizes follow ISO metric tables — tap drill diameter is nominal minus pitch for coarse threads. Wrong drill size causes loose fit or tap breakage.
Helicoil and thread inserts restore worn holes to nominal size — specify insert length for required engagement. Common in aluminium gearbox housings and repeated maintenance points on UAE plant equipment.
Common mistakes to avoid
- Ordering M12×50 without stating pitch — default coarse 1.75 assumed but fine 1.5 exists on same drawing series
- Bolt too short — threads bottom in blind hole before joint tightens
- Bolt too long — protrusion into belt path or operator zone
- Threads in shear plane on slip-critical structural joint without engineer approval
- Measuring socket screw length like hex bolt when standard differs
- Mixing coarse nut with fine bolt from mixed stock bins
RFQ checklist
- Nominal diameter (M6, M8, M10, etc.)
- Pitch: coarse (standard) or fine — state explicitly
- Length measurement reference (hex under head, socket overall)
- Grip stack thickness (mm)
- Full thread vs partial thread requirement
- Nut or tapped hole — engagement side
- Material of parent thread (steel, aluminium, cast iron)
- Standard designation (DIN 933, ISO 4014, etc.)