3/4"-10 UNC thread

3/4"-10 UNC is a coarse Unified inch thread with a 19.05 mm (0.750") major diameter and 10 threads per inch (2.540 mm pitch). Tap it with a 21/32" drill; drill 19.84 mm to pass a bolt through.

The numbers

Designation

3/4"-10 UNC

Major diameter

19.05 mm / 0.750"

Minor diameter

16.3 mm

Pitch

10 threads per inch (2.540 mm pitch)

Tap drill (75% thread)

21/32" — 16.67 mm

Clearance hole (close fit)

19.84 mm / 0.781"

Hex across flats

28.57 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used 3/4"-10 UNC reads a little under 19.05 mm — the crests are truncated by the standard, then worn and plated, so anything from about 18.7 mm up is consistent with this size.
2. Count the threads crossing one inch. This one gives 10.
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

• Structural anchor rods and holding-down bolts
• Heavy equipment and press mounting
• Large flange bolting in US practice

Other threads at this diameter