This page covers M18 Hex Bolts to ISO 4014 / ISO 4017: the full dimensional row, hole and tool sizes, proof-load data and typical applications. All values are taken from the governing standards (ISO 4014 / ISO 4017 together with ISO 273, ISO 724 and ISO 898-1 where applicable).
M18 Dimensional Row (ISO 4014 / ISO 4017)
| Size | Pitch coarse (mm) | Width across flats s (mm) | Width across corners e min (mm) | Head height k nom (mm) | Nominal diameter d (mm) | Reference standard | Notes |
|---|---|---|---|---|---|---|---|
| M18 | 2.5 | 27.0 | 30.14 | 11.5 | 18.0 | ISO 4014/4017 | Partial / fully threaded |
Spanner & Drive for M18 Hex Bolts
M18 Hex Bolts have a width across flats of 27.0 mm — fit a 27.0 mm open-ended spanner or socket. Use a 6-point socket for tight clearances and to reduce rounding; for repeated assembly choose a torque wrench so the joint preload is repeatable.
Hole Sizes for M18
| Coarse-thread pitch (ISO 724) | 2.5 mm |
|---|---|
| Through-hole / clearance (ISO 273 medium) | 20 mm |
| Tapping drill, coarse thread | 15.5 mm |
Tightening Torque for M18
| Class 8.8 (dry, ~µ 0.125) | ≈ 290 Nm |
|---|---|
| Class 10.9 (dry, ~µ 0.125) | ≈ 405 Nm |
| Class 12.9 (dry, ~µ 0.125) | ≈ 485 Nm |
Indicative dry-joint values. Lubrication can lower the required torque by 15–25%. Always confirm against the joint design, especially when going up a strength class.
Proof Load & Strength for M18
| Tensile stress area As (ISO 898-1) | 192 mm² |
|---|---|
| Proof load, class 8.8 | 115 kN |
| Proof load, class 10.9 | 159 kN |
| Proof load, class 12.9 | 186 kN |
| Min. ultimate tensile, class 8.8 | 154 kN |
Proof load = stress area × proof stress per ISO 898-1. Design joints so the working preload stays below the proof load of the selected class.
Common Applications for M18 Hex Bolts
M18 Hex Bolts are commonly specified for structural steelwork, heavy machinery foundations and large flange joints.
Installation Tips for M18 Hex Bolts
- Use a 6-point socket where access allows — 12-point sockets are more prone to rounding the corners on smaller sizes.
- On flanged or serrated variants, do NOT add a separate flat washer — the flange already spreads the load and the washer can defeat the locking serrations.
- At M18 the stretch in the bolt becomes the controlling factor — for critical joints, use angle-controlled or stretch-controlled tightening instead of pure torque.
Mating Parts for M18
For M18, pair with a M18 hex nut (ISO 4032 / DIN 934) and, where used, a M18 flat washer (ISO 7089 / DIN 125A) under the head and under the nut.