#5-40 UNC thread

#5-40 UNC is a coarse Unified inch thread with a 3.175 mm (0.125") major diameter and 40 threads per inch (0.635 mm pitch). Tap it with a #38 drill; drill 3.45 mm to pass a bolt through.

The numbers

Designation

#5-40 UNC

Major diameter

3.175 mm / 0.125"

Minor diameter

2.488 mm

Pitch

40 threads per inch (0.635 mm pitch)

Tap drill (75% thread)

#38 — 2.58 mm

Clearance hole (close fit)

3.45 mm / 0.136"

Hex across flats

7.94 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used #5-40 UNC reads a little under 3.175 mm — the crests are truncated by the standard, then worn and plated, so anything from about 2.8 mm up is consistent with this size.
2. Count the threads crossing one inch. This one gives 40.
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

• Appliance and instrument chassis screws
• Older US electrical hardware