1/4"-20 UNC thread

1/4"-20 UNC is a coarse Unified inch thread with a 6.35 mm (0.250") major diameter and 20 threads per inch (1.270 mm pitch). Tap it with a #7 drill; drill 6.76 mm to pass a bolt through.

The numbers

Designation

1/4"-20 UNC

Major diameter

6.35 mm / 0.250"

Minor diameter

4.975 mm

Pitch

20 threads per inch (1.270 mm pitch)

Tap drill (75% thread)

#7 — 5.11 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"-20 UNC 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 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

• Camera tripod sockets and quick-release plates
• Studio and video accessory mounts
• US furniture connector bolts and T-slot tables
• Workbench dog holes and hold-down hardware

The camera tripod thread. Cameras, quick-release plates and small ball heads all use 1/4"-20; the larger 3/8"-16 is used for heavier heads and studio stands.

Other threads at this diameter