1/2"-20 UNF thread

1/2"-20 UNF is a fine Unified inch thread with a 12.7 mm (0.500") major diameter and 20 threads per inch (1.270 mm pitch). Tap it with a 29/64" drill; drill 13.49 mm to pass a bolt through.

The numbers

Designation

1/2"-20 UNF

Major diameter

12.7 mm / 0.500"

Minor diameter

11.325 mm

Pitch

20 threads per inch (1.270 mm pitch)

Tap drill (75% thread)

29/64" — 11.51 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"-20 UNF 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 20.
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

Plain steel property class 8.8 or SAE grade 5, dry threads, tightened to about 65% of yield. Lubricated threads reach the same preload at 20-30% less torque. Where the manufacturer publishes a figure — spark plugs, wheel nuts, cylinder heads — use theirs.

Where this size turns up

• Automotive spindle and axle nuts
• Trailer hitch ball shanks on lighter couplings
• US fine-thread machinery bolts

Other threads at this diameter