1"-8 UNC thread

1"-8 UNC is a coarse Unified inch thread with a 25.4 mm (1.000") major diameter and 8 threads per inch (3.175 mm pitch). Tap it with a 7/8" drill; drill 26.19 mm to pass a bolt through.

The numbers

Designation

1"-8 UNC

Major diameter

25.4 mm / 1.000"

Minor diameter

21.963 mm

Pitch

8 threads per inch (3.175 mm pitch)

Tap drill (75% thread)

7/8" — 22.22 mm

Clearance hole (close fit)

26.19 mm / 1.031"

Hex across flats

38.1 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used 1"-8 UNC reads a little under 25.4 mm — the crests are truncated by the standard, then worn and plated, so anything from about 25.0 mm up is consistent with this size.
2. Count the threads crossing one inch. This one gives 8.
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 in foundations
• Heavy machine bases and press frames
• Large flange and tower connections

Other threads at this diameter