UNC Unified Coarse Thread · ANSI/ASME B1.1
1/4" - 20 UNC Thread: Pitch, Tap Drill & Dimensions
1/4" - 20 UNC is a UNC Unified Coarse Thread (ANSI/ASME B1.1) size, with a major diameter of 6.350 mm, 20 threads per inch (1.270 mm pitch). The tap drill is 5.11 mm.
| Major Diameter (inch) | 0.250 |
|---|---|
| Major Diameter (mm) | 6.350 |
| Tapping Drill Size (mm) | 5.11 |
| TPI | 20 |
| Pitch (mm) | 1.270 |
| Flank angle | 60° |
Common questions
What size drill do I use to tap 1/4" - 20 UNC?
Use a 5.11 drill. This is the standard tap drill for 1/4" - 20 UNC; for hard materials or deep threads a slightly larger drill reduces tapping torque.
What is the pitch of 1/4" - 20 UNC?
The pitch of 1/4" - 20 UNC is 1.270, meaning the thread advances that distance in one full turn.
Where can I find other UNC Unified Coarse Thread sizes?
The full UNC Unified Coarse 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.