7/8"-9 UNC thread

7/8"-9 UNC is a coarse Unified inch thread with a 22.225 mm (0.875") major diameter and 9 threads per inch (2.822 mm pitch). Tap it with a 49/64" drill; drill 23.02 mm to pass a bolt through.

The numbers

Designation

7/8"-9 UNC

Major diameter

22.225 mm / 0.875"

Minor diameter

19.17 mm

Pitch

9 threads per inch (2.822 mm pitch)

Tap drill (75% thread)

49/64" — 19.45 mm

Clearance hole (close fit)

23.02 mm / 0.906"

Hex across flats

33.34 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used 7/8"-9 UNC reads a little under 22.225 mm — the crests are truncated by the standard, then worn and plated, so anything from about 21.8 mm up is consistent with this size.
2. Count the threads crossing one inch. This one gives 9.
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

• Heavy structural anchor bolts
• Large machine base and press bolting

Other threads at this diameter