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M8 bolt size
13 mm wrench
across-flats spanner / socket size for M8
M8 bolt — automotive, machinery and furniture.
Full specification
| System | Metric coarse |
| Nominal diameter | 8 mm |
| Thread pitch | 1.25 mm |
| Wrench / spanner (across flats) | 13 mm |
| Tap drill | 6.8 mm |
| Clearance hole | 9 mm |
| Common grade / class | 8.8 |
| Typical torque | 25 Nm (18.4 ft·lb) |
| Closest other-system size | ~5/16 inch |
🔧 Torque unit converter
Pre-filled with the typical torque for M8 (8.8, dry). Edit any field to convert.
⚠️ Torque values here are general references for 8.8 on clean, dry threads. Lubrication, coatings, and the joint material change the figure significantly. For structural, automotive, or safety-critical work, always use the torque specified by the manufacturer or engineer.
About M8
M8 is extremely common in automotive and machinery work, and one of the most searched sizes.
A 6.8 mm tap drill is correct for M8 coarse — a 7 mm drill also works in a pinch.
Frequently asked questions
- What size wrench or spanner fits M8?
- M8 bolt uses a 13 mm wrench or spanner, measured across the flats of the hex head.
- What is the torque for M8?
- A typical tightening torque for M8 bolt in 8.8 on dry threads is about 25 Nm (18.4 ft·lb). Always confirm against the manufacturer spec for important joints.
- What thread pitch and tap drill does M8 use?
- M8 bolt has a thread pitch of 1.25 mm and uses a 6.8 mm tap drill, with a 9 mm clearance hole.