3/4"-20 UNEF thread

3/4"-20 UNEF is a extra-fine Unified inch thread with a 19.05 mm (0.750") major diameter and 20 threads per inch (1.270 mm pitch). Tap it with a 45/64" drill; drill 19.84 mm to pass a bolt through.

The numbers

Designation

3/4"-20 UNEF

Major diameter

19.05 mm / 0.750"

Minor diameter

17.675 mm

Pitch

20 threads per inch (1.270 mm pitch)

Tap drill (75% thread)

45/64" — 17.86 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"-20 UNEF 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 20.
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

No torque figure is given for this size. Small and fine threads like this are generally not supplied to a strength grade, and in most assemblies the tapped material strips before the fastener yields — so a published bolt torque would be the wrong limit to work to. Tighten to snug plus a small turn, or use the figure the manufacturer of the part specifies.

Where this size turns up

• Instrument bushings and thin-wall adaptors
• Aerospace fine-thread fittings

Other threads at this diameter