UNF Unified Fine Thread · ANSI/ASME B1.1
1/4" - 28 UNF Thread: Pitch, Tap Drill & Dimensions
1/4" - 28 UNF is a UNF Unified Fine Thread (ANSI/ASME B1.1) size, with a major diameter of 6.350 mm, 28 threads per inch (0.907 mm pitch). The tap drill is 5.50 mm.
| Major Diameter (inch) | 0.250 |
|---|---|
| Major Diameter (mm) | 6.350 |
| Tapping Drill Size (mm) | 5.50 |
| TPI | 28 |
| Pitch (mm) | 0.907 |
| Flank angle | 60° |
Common questions
What size drill do I use to tap 1/4" - 28 UNF?
Use a 5.50 drill. This is the standard tap drill for 1/4" - 28 UNF; for hard materials or deep threads a slightly larger drill reduces tapping torque.
What is the pitch of 1/4" - 28 UNF?
The pitch of 1/4" - 28 UNF is 0.907, meaning the thread advances that distance in one full turn.
Where can I find other UNF Unified Fine Thread sizes?
The full UNF Unified Fine Thread table, with every size and a live filter, is on its standard page, and the Thread Finder searches all 73 standards at once.
Reference values; computed values use the ISO basic profile. Verify against the current standard before production use.