3/8"-24 UNF thread

3/8"-24 UNF is a fine Unified inch thread with a 9.525 mm (0.375") major diameter and 24 threads per inch (1.058 mm pitch). Tap it with a Q drill; drill 10.08 mm to pass a bolt through.

The numbers

Designation

3/8"-24 UNF

Major diameter

9.525 mm / 0.375"

Minor diameter

8.379 mm

Pitch

24 threads per inch (1.058 mm pitch)

Tap drill (75% thread)

Q — 8.43 mm

Clearance hole (close fit)

10.08 mm / 0.397"

Hex across flats

14.29 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used 3/8"-24 UNF reads a little under 9.525 mm — the crests are truncated by the standard, then worn and plated, so anything from about 9.1 mm up is consistent with this size.
2. Count the threads crossing one inch. This one gives 24.
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

• US inverted-flare brake line fittings for 3/16-inch tube
• Automotive fine-thread bolts and studs
• Aircraft hardware

Also the thread on US inverted-flare brake line fittings for 3/16-inch tube, which is why a brake union and a bolt can look interchangeable and are not.

Other threads at this diameter