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1/4-28 tap drill size
Use a #3 drill (.2130 in / 5.41 mm) to tap 1/4-28 UNF. That gives about 80% thread engagement. Closest metric drill: 5.4 mm.
1/4-28 thread specifications
| Thread | 1/4-28 UNF (fine) |
|---|---|
| Major (nominal) diameter | .2500 in / 6.35 mm |
| Threads per inch | 28 (pitch 0.907 mm) |
| Theoretical drill for 75% thread | .2152 in (5.47 mm) |
| Recommended tap drill | #3 = .2130 in |
Alternative drills and % of thread
Any of these drills will tap 1/4-28. Smaller drills give more thread but make tapping harder and raise the risk of breaking the tap, especially in stainless steel and in small sizes. 60–65% thread is common in tough materials. 75% is the general-purpose standard.
Tips for tapping 1/4-28
- Use cutting fluid, and with a hand tap back off about half a turn every turn or two to break the chip. Aluminium and soft materials are the exception: they tap cleanly with spiral-point taps.
- For blind holes, drill deeper than the thread you need. Allow at least 3–4 pitches beyond the last full thread for a plug tap (0.143" for 28 TPI).
- Thread engagement of 1–1.5 × the diameter (6.3–9.5 mm) is enough for most steel parts. Softer materials need more.