5/8"-11 UNC thread

5/8"-11 UNC is a coarse Unified inch thread with a 15.875 mm (0.625") major diameter and 11 threads per inch (2.309 mm pitch). Tap it with a 17/32" drill; drill 16.67 mm to pass a bolt through.

The numbers

Designation

5/8"-11 UNC

Major diameter

15.875 mm / 0.625"

Minor diameter

13.375 mm

Pitch

11 threads per inch (2.309 mm pitch)

Tap drill (75% thread)

17/32" — 13.49 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"-11 UNC 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 11.
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

• Concrete anchor bolts and holding-down studs
• Trailer hitch balls and towing hardware
• Heavy machinery mounting

Other threads at this diameter