M18 Bolt Specifications

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Reviewed by MLC Editorial Team | 16 August 2026

Verified by Rachel Mayfield, Supply Chain Analyst - May 2026

Close-up image of three steel bolts standing upright against a contrasting yellow background. — 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

References

ISO 261:1998 - ISO metric screw threads
ISO 898-1:2013 - Mechanical properties of fasteners
VDI 2230:2015 - Calculation of bolted joints

M18 bolts and tools on Amazon

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Buy M18 hex bolts (DIN 933) on Amazon →
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Buy Torque wrench for M18 (300Nm) on Amazon →

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Frequently Asked Questions

What is the thread pitch of an M18 bolt?

A standard M18 bolt has a coarse thread pitch of 2.5 mm per ISO 261. The fine pitch version is M18 x 2.0 / 1.5. If a drawing says M18 with no pitch specified, it means M18 x 2.5 coarse.

What is the tap drill size for M18?

For M18 x 2.5 coarse pitch with 75% thread engagement, drill 15.5 mm. For fine pitch, drill 16.0 / 16.5 mm. These values give a good balance of thread strength and tapping ease.

What torque is an M18 bolt rated for?

For a dry M18 property class 8.8 bolt, the standard torque is 285 Nm. Class 10.9 takes 405 Nm, class 12.9 takes 475 Nm. Values are per VDI 2230 at 90% proof load with nut factor K=0.20. Reduce by 20-25% if threads are lubricated.

What spanner size fits an M18 bolt?

An M18 hex head bolt (ISO 4014) has a 27.0 mm across-flats (A/F) dimension, so you need a 27.0 mm spanner or socket. An M18 socket head cap screw uses a 14.0 mm hex key (Allen key).

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