1/4"-32 UNEF thread

1/4"-32 UNEF is a extra-fine Unified inch thread with a 6.35 mm (0.250") major diameter and 32 threads per inch (0.794 mm pitch). Tap it with a 7/32" drill; drill 6.76 mm to pass a bolt through.

The numbers

Designation

1/4"-32 UNEF

Major diameter

6.35 mm / 0.250"

Minor diameter

5.491 mm

Pitch

32 threads per inch (0.794 mm pitch)

Tap drill (75% thread)

7/32" — 5.56 mm

Clearance hole (close fit)

6.76 mm / 0.266"

Hex across flats

11.11 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used 1/4"-32 UNEF reads a little under 6.35 mm — the crests are truncated by the standard, then worn and plated, so anything from about 5.9 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

• Thin-wall tube and instrument fittings
• Aerospace adjusting hardware

Other threads at this diameter