M20 Bolt Specifications
Verified by Rachel Mayfield, Supply Chain Analyst - April 2026
Every dimension, torque value, and thread specification for the M20 metric bolt in one reference card. M20 Bolt Specifications M20 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 | 20.0 mm | ISO 261 |
| Coarse pitch | 2.5 mm | ISO 261 |
| Fine pitch | 2.0 / 1.5 mm | ISO 261 |
| Tap drill (coarse, 75% engagement) | 17.5 mm | ISO 2306 |
| Tap drill (fine) | 18.0 / 18.5 mm | ISO 2306 |
| Clearance hole - close | 21.0 mm | ISO 273 |
| Clearance hole - medium | 22.0 mm | ISO 273 |
| Clearance hole - coarse | 24.0 mm | ISO 273 |
| Hex head A/F (spanner size) | 30.0 mm | ISO 4014 |
| Hex head height | 12.5 mm | ISO 4014 |
| Socket head cap screw head dia | 30.0 mm | ISO 4762 |
| Socket head hex key size | 17.0 mm | ISO 4762 |
| Tensile stress area | 245 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 | 142.1 kN | ISO 898-1 |
| Proof load 10.9 | 203.4 kN | ISO 898-1 |
| Torque dry, class 8.8 | 405 Nm | VDI 2230 |
| Torque dry, class 10.9 | 570 Nm | VDI 2230 |
| Torque dry, class 12.9 | 670 Nm | VDI 2230 |
M20 thread pitch
Standard M20 bolts use a 2.5 mm coarse pitch. Fine pitch M20 x 2.0 is used where vibration resistance matters or in thin-wall components. Coarse pitch is the default - if a drawing says M20 with no pitch specified, it means M20 x 2.5 coarse.
M20 tap drill size
For M20 x 2.5 coarse pitch with 75% thread engagement, drill 17.5 mm. For fine pitch, drill 18.0 / 18.5 mm. Smaller drills give more engagement but increase tap breakage risk without meaningful strength gain.
M20 clearance hole
For a clearance hole where the bolt passes through, use 21.0 mm (close fit), 22.0 mm (medium), or 24.0 mm (coarse) per ISO 273.
M20 torque values
Dry M20 property class 8.8: torque to 405 Nm. Class 10.9: 570 Nm. Class 12.9: 670 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.
M20 spanner and hex key
An M20 hex head bolt (ISO 4014) has 30.0 mm across flats, so you need a 30.0 mm spanner or socket. M20 socket head cap screws (ISO 4762) have 30.0 mm head diameter and need a 17.0 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
Frequently Asked Questions
What is the thread pitch of an M20 bolt?
A standard M20 bolt has a coarse thread pitch of 2.5 mm per ISO 261. The fine pitch version is M20 x 2.0 / 1.5. If a drawing says M20 with no pitch specified, it means M20 x 2.5 coarse.
What is the tap drill size for M20?
For M20 x 2.5 coarse pitch with 75% thread engagement, drill 17.5 mm. For fine pitch, drill 18.0 / 18.5 mm. These values give a good balance of thread strength and tapping ease.
What torque is an M20 bolt rated for?
For a dry M20 property class 8.8 bolt, the standard torque is 405 Nm. Class 10.9 takes 570 Nm, class 12.9 takes 670 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 M20 bolt?
An M20 hex head bolt (ISO 4014) has a 30.0 mm across-flats (A/F) dimension, so you need a 30.0 mm spanner or socket. An M20 socket head cap screw uses a 17.0 mm hex key (Allen key).