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1/4-20 tap drill size
Use a #7 drill (.2010 in / 5.11 mm) to tap 1/4-20 UNC. That gives about 75% thread engagement. Closest metric drill: 5.1 mm.
1/4-20 thread specifications
| Thread | 1/4-20 UNC (coarse) |
|---|---|
| Major (nominal) diameter | .2500 in / 6.35 mm |
| Threads per inch | 20 (pitch 1.27 mm) |
| Theoretical drill for 75% thread | .2013 in (5.11 mm) |
| Recommended tap drill | #7 = .2010 in |
Alternative drills and % of thread
Any of these drills will tap 1/4-20. 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.
| Drill | Inch | mm | % thread |
|---|---|---|---|
| #9 | .1960 in | 4.98 | 83% |
| #8 | .1990 in | 5.05 | 79% |
| #7 | .2010 in | 5.11 | 75% |
| 13/64 | .2031 in | 5.16 | 72% |
| #6 | .2040 in | 5.18 | 71% |
| #5 | .2055 in | 5.22 | 69% |
| #4 | .2090 in | 5.31 | 63% |
| #3 | .2130 in | 5.41 | 57% |
Tips for tapping 1/4-20
- 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 (0.200" for 20 TPI).
- Thread engagement of 1–1.5 × the diameter (6.3–9.5 mm) is enough for most steel parts. Softer materials need more.