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5/8-11 tap drill size
Use a 17/32 drill (.5313 in / 13.49 mm) to tap 5/8-11 UNC. That gives about 79% thread engagement. Closest metric drill: 13.5 mm.
5/8-11 thread specifications
| Thread | 5/8-11 UNC (coarse) |
|---|---|
| Major (nominal) diameter | .6250 in / 15.88 mm |
| Threads per inch | 11 (pitch 2.309 mm) |
| Theoretical drill for 75% thread | .5364 in (13.62 mm) |
| Recommended tap drill | 17/32 = .5313 in |
Alternative drills and % of thread
Any of these drills will tap 5/8-11. 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 5/8-11
- 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 (0.364" for 11 TPI).
- Thread engagement of 1–1.5 × the diameter (15.9–23.8 mm) is enough for most steel parts. Softer materials need more.