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1/2-20 tap drill size
Use a 29/64 drill (.4531 in / 11.51 mm) to tap 1/2-20 UNF. That gives about 72% thread engagement. Closest metric drill: 11.5 mm.
1/2-20 thread specifications
| Thread | 1/2-20 UNF (fine) |
|---|---|
| Major (nominal) diameter | .5000 in / 12.70 mm |
| Threads per inch | 20 (pitch 1.27 mm) |
| Theoretical drill for 75% thread | .4513 in (11.46 mm) |
| Recommended tap drill | 29/64 = .4531 in |
Alternative drills and % of thread
Any of these drills will tap 1/2-20. 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 |
|---|---|---|---|
| 29/64 | .4531 in | 11.51 | 72% |
Tips for tapping 1/2-20
- 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 (12.7–19.0 mm) is enough for most steel parts. Softer materials need more.