5/16"-18 UNC thread

5/16"-18 UNC is a coarse Unified inch thread with a 7.938 mm (0.313") major diameter and 18 threads per inch (1.411 mm pitch). Tap it with a F drill; drill 8.43 mm to pass a bolt through.

The numbers

Designation

5/16"-18 UNC

Major diameter

7.938 mm / 0.313"

Minor diameter

6.41 mm

Pitch

18 threads per inch (1.411 mm pitch)

Tap drill (75% thread)

F — 6.53 mm

Clearance hole (close fit)

8.43 mm / 0.332"

Hex across flats

12.7 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used 5/16"-18 UNC reads a little under 7.938 mm — the crests are truncated by the standard, then worn and plated, so anything from about 7.5 mm up is consistent with this size.
2. Count the threads crossing one inch. This one gives 18.
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 automotive body and bracket bolts
• Trailer and utility hardware
• Deck and framing connector bolts

Other threads at this diameter