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Every dimension, torque value, and thread specification for the M8 metric bolt in one reference card. M8 Bolt Specifications M8 is one of the most common metric fastener sizes in UK engineering, used across general machine assembly, motor brackets and structural fixings.
| Property |
Value |
Standard |
| Nominal diameter |
8.0 mm |
ISO 261 |
| Coarse pitch |
1.25 mm |
ISO 261 |
| Fine pitch |
1.0 / 0.75 mm |
ISO 261 |
| Tap drill (coarse, 75% engagement) |
6.8 mm |
ISO 2306 |
| Tap drill (fine) |
7.0 / 7.25 mm |
ISO 2306 |
| Clearance hole - close |
8.4 mm |
ISO 273 |
| Clearance hole - medium |
9.0 mm |
ISO 273 |
| Clearance hole - coarse |
10.0 mm |
ISO 273 |
| Hex head A/F (spanner size) |
13.0 mm |
ISO 4014 |
| Hex head height |
5.3 mm |
ISO 4014 |
| Socket head cap screw head dia |
13.0 mm |
ISO 4762 |
| Socket head hex key size |
6.0 mm |
ISO 4762 |
| Tensile stress area |
36.6 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 |
21.2 kN |
ISO 898-1 |
| Proof load 10.9 |
30.4 kN |
ISO 898-1 |
| Torque dry, class 8.8 |
25 Nm |
VDI 2230 |
| Torque dry, class 10.9 |
35 Nm |
VDI 2230 |
| Torque dry, class 12.9 |
41 Nm |
VDI 2230 |
M8 thread pitch
Standard M8 bolts use a 1.25 mm coarse pitch. Fine pitch M8 x 1.0 is used where vibration resistance matters or in thin-wall components. Coarse pitch is the default - if a drawing says M8 with no pitch specified, it means M8 x 1.25 coarse.
M8 tap drill size
For M8 x 1.25 coarse pitch with 75% thread engagement, drill 6.8 mm. For fine pitch, drill 7.0 / 7.25 mm. Smaller drills give more engagement but increase tap breakage risk without meaningful strength gain.
M8 clearance hole
For a clearance hole where the bolt passes through, use 8.4 mm (close fit), 9.0 mm (medium), or 10.0 mm (coarse) per ISO 273.
M8 torque values
Dry M8 property class 8.8: torque to 25 Nm. Class 10.9: 35 Nm. Class 12.9: 41 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.
M8 spanner and hex key
An M8 hex head bolt (ISO 4014) has 13.0 mm across flats, so you need a 13.0 mm spanner or socket. M8 socket head cap screws (ISO 4762) have 13.0 mm head diameter and need a 6.0 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
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Looking for machine screw head sizes rather than bolt data? The M8 screw dimensions card covers pan, countersunk, socket cap, button and cheese heads with drive sizes and DIN vs ISO differences.
Frequently Asked Questions
What is the thread pitch of an M8 bolt?
A standard M8 bolt has a coarse thread pitch of 1.25 mm per ISO 261. The fine pitch version is M8 x 1.0 / 0.75. If a drawing says M8 with no pitch specified, it means M8 x 1.25 coarse.
What is the tap drill size for M8?
For M8 x 1.25 coarse pitch with 75% thread engagement, drill 6.8 mm. For fine pitch, drill 7.0 / 7.25 mm. These values give a good balance of thread strength and tapping ease.
What torque is an M8 bolt rated for?
For a dry M8 property class 8.8 bolt, the standard torque is 25 Nm. Class 10.9 takes 35 Nm, class 12.9 takes 41 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 M8 bolt?
An M8 hex head bolt (ISO 4014) has a 13.0 mm across-flats (A/F) dimension, so you need a 13.0 mm spanner or socket. An M8 socket head cap screw uses a 6.0 mm hex key (Allen key).
What is the M8 bolt size in mm?
An M8 bolt has a nominal diameter of 8.0 mm, which is the major thread diameter measured across the outside of the thread. The M8 standard coarse pitch is 1.25 mm per ISO 261. The hex head measures 13.0 mm across flats per ISO 4014, so an M8 bolt takes a 13.0 mm spanner or socket.
What are the M8 bolt dimensions?
The M8 bolt has an 8.0 mm nominal diameter and a 1.25 mm coarse pitch per ISO 261. The hex head is 13.0 mm across flats with a 5.3 mm head height per ISO 4014. The tensile stress area of an M8 thread is 36.6 mm squared per ISO 898-1, and the clearance hole ranges from 8.4 mm close to 10.0 mm coarse per ISO 273.
What is the M8 standard pitch?
The M8 standard pitch is 1.25 mm, the ISO 261 coarse pitch used by default. If a drawing specifies M8 with no pitch, it means M8 x 1.25 coarse. Fine pitch options for the M8 thread are 1.0 mm and 0.75 mm, chosen where vibration resistance matters or in thin-wall components.
What is the M8 bolt head size?
An M8 hex head bolt has a head measuring 13.0 mm across flats per ISO 4014, so it needs a 13.0 mm spanner or socket. The M8 hex head height is 5.3 mm. An M8 socket head cap screw per ISO 4762 has a 13.0 mm head diameter and takes a 6.0 mm hex key.
What is the M8 bolt size?
The M8 bolt size refers to an 8.0 mm nominal thread diameter per ISO 261. The default M8 coarse pitch is 1.25 mm. For tapping an M8 thread at 75 percent engagement, drill 6.8 mm per ISO 2306, and for a clearance hole use 8.4 mm close, 9.0 mm medium or 10.0 mm coarse per ISO 273.