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5/8-18 UNF bolt size
15/16 in wrench
across-flats spanner / socket size for 5/8-18
5/8-18 UNF bolt — heavy machinery and axles.
Full specification
| System | Imperial UNF |
| Nominal diameter | 5/8 in (15.88 mm) |
| Thread pitch | 18 TPI (fine) |
| Wrench / spanner (across flats) | 15/16 in |
| Tap drill | 37/64 in |
| Clearance hole | 41/64 in |
| Common grade / class | Grade 5 |
| Typical torque | 225 Nm (166 ft·lb) |
| Closest other-system size | ~5/8-11 coarse |
🔧 Torque unit converter
Pre-filled with the typical torque for 5/8-18 (Grade 5, dry). Edit any field to convert.
⚠️ Torque values here are general references for Grade 5 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 5/8-18
5/8-18 fine thread is used on heavy axles and machine spindles.
Use a torque wrench — fine threads reach high clamp load at lower torque.
Frequently asked questions
- What size wrench or spanner fits 5/8-18?
- 5/8-18 UNF bolt uses a 15/16 in wrench or spanner, measured across the flats of the hex head.
- What is the torque for 5/8-18?
- A typical tightening torque for 5/8-18 UNF bolt in Grade 5 on dry threads is about 225 Nm (166 ft·lb). Always confirm against the manufacturer spec for important joints.
- What thread pitch and tap drill does 5/8-18 use?
- 5/8-18 UNF bolt has a thread pitch of 18 TPI (fine) and uses a 37/64 in tap drill, with a 41/64 in clearance hole.