1/2"-28 UNEF thread

1/2"-28 UNEF is a extra-fine Unified inch thread with a 12.7 mm (0.500") major diameter and 28 threads per inch (0.907 mm pitch). Tap it with a 15/32" drill; drill 13.49 mm to pass a bolt through.

The numbers

Designation

1/2"-28 UNEF

Major diameter

12.7 mm / 0.500"

Minor diameter

11.718 mm

Pitch

28 threads per inch (0.907 mm pitch)

Tap drill (75% thread)

15/32" — 11.91 mm

Clearance hole (close fit)

13.49 mm / 0.531"

Hex across flats

19.05 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used 1/2"-28 UNEF reads a little under 12.7 mm — the crests are truncated by the standard, then worn and plated, so anything from about 12.3 mm up is consistent with this size.
2. Count the threads crossing one inch. This one gives 28.
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 muzzle devices on some .22 and 9 mm barrels
• Thin-wall tube fittings

Other threads at this diameter