1/4"-28 UNF thread

1/4"-28 UNF is a fine Unified inch thread with a 6.35 mm (0.250") major diameter and 28 threads per inch (0.907 mm pitch). Tap it with a #3 drill; drill 6.76 mm to pass a bolt through.

The numbers

Designation

1/4"-28 UNF

Major diameter

6.35 mm / 0.250"

Minor diameter

5.368 mm

Pitch

28 threads per inch (0.907 mm pitch)

Tap drill (75% thread)

#3 — 5.41 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"-28 UNF 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 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

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

• Grease nipples and zerk fittings
• US automotive fine-thread bolts
• Aircraft AN and MS hardware

Other threads at this diameter