3/8"-16 UNC thread

3/8"-16 UNC is a coarse Unified inch thread with a 9.525 mm (0.375") major diameter and 16 threads per inch (1.587 mm pitch). Tap it with a 5/16" drill; drill 10.08 mm to pass a bolt through.

The numbers

Designation

3/8"-16 UNC

Major diameter

9.525 mm / 0.375"

Minor diameter

7.806 mm

Pitch

16 threads per inch (1.587 mm pitch)

Tap drill (75% thread)

5/16" — 7.94 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"-16 UNC 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 16.
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

• Studio tripod heads and light stand spigots
• Microphone stand boom clamps and adaptors
• US structural and machinery bolting
• Lifting eye bolts

The large tripod and stand thread. Studio heads, microphone stands, light stands and boom arms use 3/8"-16; adaptor bushings step it down to 1/4"-20.

Other threads at this diameter