#6-40 UNF thread

#6-40 UNF is a fine Unified inch thread with a 3.505 mm (0.138") major diameter and 40 threads per inch (0.635 mm pitch). Tap it with a #33 drill; drill 3.8 mm to pass a bolt through.

The numbers

Designation

#6-40 UNF

Major diameter

3.505 mm / 0.138"

Minor diameter

2.818 mm

Pitch

40 threads per inch (0.635 mm pitch)

Tap drill (75% thread)

#33 — 2.87 mm

Clearance hole (close fit)

3.8 mm / 0.150"

Hex across flats

7.94 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used #6-40 UNF reads a little under 3.505 mm — the crests are truncated by the standard, then worn and plated, so anything from about 3.1 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

• Firearm scope rings and base screws
• Precision instrument hardware

Other threads at this diameter