M42 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 M42 metric bolt in one reference card. M42 Bolt Specifications M42 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	42.0 mm	ISO 261
Coarse pitch	4.5 mm	ISO 261
Fine pitch	3.0 mm	ISO 261
Tap drill (coarse, 75% engagement)	37.5 mm	ISO 2306
Tap drill (fine)	39.0 mm	ISO 2306
Clearance hole - close	43.0 mm	ISO 273
Clearance hole - medium	45.0 mm	ISO 273
Clearance hole - coarse	48.0 mm	ISO 273
Hex head A/F (spanner size)	65.0 mm	ISO 4014
Hex head height	26.0 mm	ISO 4014
Socket head cap screw head dia	63.0 mm	ISO 4762
Socket head hex key size	32.0 mm	ISO 4762
Tensile stress area	1121 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	650.2 kN	ISO 898-1
Proof load 10.9	930.4 kN	ISO 898-1
Torque dry, class 8.8	3870 Nm	VDI 2230
Torque dry, class 10.9	5545 Nm	VDI 2230
Torque dry, class 12.9	6475 Nm	VDI 2230

M42 thread pitch

Standard M42 bolts use a 4.5 mm coarse pitch. Fine pitch M42 x 3.0 is used where vibration resistance matters or in thin-wall components. Coarse pitch is the default - if a drawing says M42 with no pitch specified, it means M42 x 4.5 coarse.

M42 tap drill size

For M42 x 4.5 coarse pitch with 75% thread engagement, drill 37.5 mm. For fine pitch, drill 39.0 mm. Smaller drills give more engagement but increase tap breakage risk without meaningful strength gain.

M42 clearance hole

For a clearance hole where the bolt passes through, use 43.0 mm (close fit), 45.0 mm (medium), or 48.0 mm (coarse) per ISO 273.

M42 torque values

Dry M42 property class 8.8: torque to 3870 Nm. Class 10.9: 5545 Nm. Class 12.9: 6475 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.

M42 spanner and hex key

An M42 hex head bolt (ISO 4014) has 65.0 mm across flats, so you need a 65.0 mm spanner or socket. M42 socket head cap screws (ISO 4762) have 63.0 mm head diameter and need a 32.0 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

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Frequently Asked Questions

What is the thread pitch of an M42 bolt?

A standard M42 bolt has a coarse thread pitch of 4.5 mm per ISO 261. The fine pitch version is M42 x 3.0. If a drawing says M42 with no pitch specified, it means M42 x 4.5 coarse.

What is the tap drill size for M42?

For M42 x 4.5 coarse pitch with 75% thread engagement, drill 37.5 mm. For fine pitch, drill 39.0 mm. These values give a good balance of thread strength and tapping ease.

What torque is an M42 bolt rated for?

For a dry M42 property class 8.8 bolt, the standard torque is 3870 Nm. Class 10.9 takes 5545 Nm, class 12.9 takes 6475 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 M42 bolt?

An M42 hex head bolt (ISO 4014) has a 65.0 mm across-flats (A/F) dimension, so you need a 65.0 mm spanner or socket. An M42 socket head cap screw uses a 32.0 mm hex key (Allen key).

Independent UK engineering trade publication. Company profiles cross-referenced with Companies House.

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