5/8"-18 UNF thread

5/8"-18 UNF is a fine Unified inch thread with a 15.875 mm (0.625") major diameter and 18 threads per inch (1.411 mm pitch). Tap it with a 37/64" drill; drill 16.67 mm to pass a bolt through.

The numbers

Designation

5/8"-18 UNF

Major diameter

15.875 mm / 0.625"

Minor diameter

14.347 mm

Pitch

18 threads per inch (1.411 mm pitch)

Tap drill (75% thread)

37/64" — 14.68 mm

Clearance hole (close fit)

16.67 mm / 0.656"

Hex across flats

23.81 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used 5/8"-18 UNF reads a little under 15.875 mm — the crests are truncated by the standard, then worn and plated, so anything from about 15.5 mm up is consistent with this size.
2. Count the threads crossing one inch. This one gives 18.
3. A thread pitch gauge settles it in seconds and costs very little. Guessing between a coarse and a fine thread of the same diameter is the usual way a thread gets stripped.

Tightening torque

Plain steel property class 8.8 or SAE grade 5, dry threads, tightened to about 65% of yield. Lubricated threads reach the same preload at 20-30% less torque. Where the manufacturer publishes a figure — spark plugs, wheel nuts, cylinder heads — use theirs.

Where this size turns up

• Automotive axle and hub nuts
• Heavy fine-thread machinery studs

Other threads at this diameter