Verified by Rachel Mayfield, Supply Chain Analyst - May 2026
Photo by Győző Mórocz on Pexels
Every dimension, torque value, and thread specification for the M18 metric bolt in one reference card. M18 Bolt Specifications M18 is one of the most common metric fastener sizes in UK engineering, used across heavy structural and high-load joints.
| Property |
Value |
Standard |
| Nominal diameter |
18.0 mm |
ISO 261 |
| Coarse pitch |
2.5 mm |
ISO 261 |
| Fine pitch |
2.0 / 1.5 mm |
ISO 261 |
| Tap drill (coarse, 75% engagement) |
15.5 mm |
ISO 2306 |
| Tap drill (fine) |
16.0 / 16.5 mm |
ISO 2306 |
| Clearance hole - close |
19.0 mm |
ISO 273 |
| Clearance hole - medium |
20.0 mm |
ISO 273 |
| Clearance hole - coarse |
21.0 mm |
ISO 273 |
| Hex head A/F (spanner size) |
27.0 mm |
ISO 4014 |
| Hex head height |
11.5 mm |
ISO 4014 |
| Socket head cap screw head dia |
27.0 mm |
ISO 4762 |
| Socket head hex key size |
14.0 mm |
ISO 4762 |
| Tensile stress area |
192 mm^2 |
ISO 898-1 |
| Minimum tensile strength 8.8 |
800 MPa |
ISO 898-1 |
| Minimum tensile strength 10.9 |
1040 MPa |
ISO 898-1 |
| Proof load 8.8 |
111.4 kN |
ISO 898-1 |
| Proof load 10.9 |
159.4 kN |
ISO 898-1 |
| Torque dry, class 8.8 |
285 Nm |
VDI 2230 |
| Torque dry, class 10.9 |
405 Nm |
VDI 2230 |
| Torque dry, class 12.9 |
475 Nm |
VDI 2230 |
M18 thread pitch
Standard M18 bolts use a 2.5 mm coarse pitch. Fine pitch M18 x 2.0 is used where vibration resistance matters or in thin-wall components. Coarse pitch is the default - if a drawing says M18 with no pitch specified, it means M18 x 2.5 coarse.
M18 tap drill size
For M18 x 2.5 coarse pitch with 75% thread engagement, drill 15.5 mm. For fine pitch, drill 16.0 / 16.5 mm. Smaller drills give more engagement but increase tap breakage risk without meaningful strength gain.
M18 clearance hole
For a clearance hole where the bolt passes through, use 19.0 mm (close fit), 20.0 mm (medium), or 21.0 mm (coarse) per ISO 273.
M18 torque values
Dry M18 property class 8.8: torque to 285 Nm. Class 10.9: 405 Nm. Class 12.9: 475 Nm. These values target 90% proof load with K=0.20 dry thread friction. If threads are oiled or coated, reduce by 20-25%. For critical joints, always verify against the fastener manufacturer's datasheet.
M18 spanner and hex key
An M18 hex head bolt (ISO 4014) has 27.0 mm across flats, so you need a 27.0 mm spanner or socket. M18 socket head cap screws (ISO 4762) have 27.0 mm head diameter and need a 14.0 mm hex key (Allen key).
Related standards
- ISO 261 - ISO general-purpose metric screw threads
- ISO 273 - Fasteners: clearance holes for bolts and screws
- ISO 898-1 - Mechanical properties of carbon steel fasteners
- ISO 4014 - Hexagon head bolts, product grades A and B
- ISO 4762 - Hexagon socket head cap screws
- VDI 2230 - Systematic calculation of bolted joints
M18 bolts and tools on Amazon
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