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1/2-13 tap drill size
Use a 27/64 drill (.4219 in / 10.72 mm) to tap 1/2-13 UNC. That gives about 78% thread engagement. Closest metric drill: 10.7 mm.
1/2-13 thread specifications
| Thread | 1/2-13 UNC (coarse) |
|---|---|
| Major (nominal) diameter | .5000 in / 12.70 mm |
| Threads per inch | 13 (pitch 1.954 mm) |
| Theoretical drill for 75% thread | .4251 in (10.80 mm) |
| Recommended tap drill | 27/64 = .4219 in |
Alternative drills and % of thread
Any of these drills will tap 1/2-13. 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 1/2-13
- 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 (0.308" for 13 TPI).
- Thread engagement of 1–1.5 × the diameter (12.7–19.0 mm) is enough for most steel parts. Softer materials need more.