5/8"-27 UNS thread

5/8"-27 UNS is a extra-fine Unified inch thread with a 15.875 mm (0.625") major diameter and 27 threads per inch (0.941 mm pitch). Tap it with a 15 mm drill; drill 16.67 mm to pass a bolt through.

The numbers

Designation

5/8"-27 UNS

Major diameter

15.875 mm / 0.625"

Minor diameter

14.857 mm

Pitch

27 threads per inch (0.941 mm pitch)

Tap drill (75% thread)

15 mm — 15 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"-27 UNS 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 27.
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

No torque figure is given for this size. Small and fine threads like this are generally not supplied to a strength grade, and in most assemblies the tapped material strips before the fastener yields — so a published bolt torque would be the wrong limit to work to. Tighten to snug plus a small turn, or use the figure the manufacturer of the part specifies.

Where this size turns up

• Microphone stand and boom arm threads
• Microphone clips, shock mounts and adaptors
• Studio and stage stand accessories

A unified special that exists almost entirely for audio: microphone stands, clips and boom arms. Outside the US, stands are often 3/8"-16 with a 5/8"-27 adaptor in the box. The tap drill is the awkward part, because 27 threads per inch does not line up with the fractional drill series: 75% engagement wants 0.589 in and no fraction hits it. A 15 mm bit (0.591 in) is the nearest stocked drill and leaves about 73%. Tap sellers often ship 37/64 in (0.578 in), which is close to a full thread and much harder to cut, and some charts list 19/32 in (0.594 in), which leaves only about 65%.

Other threads at this diameter