#8-32 UNC thread

#8-32 UNC is a coarse Unified inch thread with a 4.166 mm (0.164") major diameter and 32 threads per inch (0.794 mm pitch). Tap it with a #29 drill; drill 4.5 mm to pass a bolt through.

The numbers

Designation

#8-32 UNC

Major diameter

4.166 mm / 0.164"

Minor diameter

3.306 mm

Pitch

32 threads per inch (0.794 mm pitch)

Tap drill (75% thread)

#29 — 3.45 mm

Clearance hole (close fit)

4.5 mm / 0.177"

Hex across flats

8.73 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used #8-32 UNC reads a little under 4.166 mm — the crests are truncated by the standard, then worn and plated, so anything from about 3.8 mm up is consistent with this size.
2. Count the threads crossing one inch. This one gives 32.
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

• US light fixture and ceiling rose mounting
• Junction box bracket and cover screws
• Appliance chassis and bracket hardware