#12-24 UNC thread

#12-24 UNC is a coarse Unified inch thread with a 5.486 mm (0.216") major diameter and 24 threads per inch (1.058 mm pitch). Tap it with a #16 drill; drill 5.79 mm to pass a bolt through.

The numbers

Designation

#12-24 UNC

Major diameter

5.486 mm / 0.216"

Minor diameter

4.341 mm

Pitch

24 threads per inch (1.058 mm pitch)

Tap drill (75% thread)

#16 — 4.5 mm

Clearance hole (close fit)

5.79 mm / 0.228"

Hex across flats

11.11 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used #12-24 UNC reads a little under 5.486 mm — the crests are truncated by the standard, then worn and plated, so anything from about 5.1 mm up is consistent with this size.
2. Count the threads crossing one inch. This one gives 24.
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

• 19-inch server rack rails in older US equipment
• Heavy sheet-metal and enclosure hardware