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9/16-18 tap drill size
Use a 33/64 drill (.5156 in / 13.10 mm) to tap 9/16-18 UNF. That gives about 65% thread engagement. Closest metric drill: 13 mm.
9/16-18 thread specifications
| Thread | 9/16-18 UNF (fine) |
|---|---|
| Major (nominal) diameter | .5625 in / 14.29 mm |
| Threads per inch | 18 (pitch 1.411 mm) |
| Theoretical drill for 75% thread | .5084 in (12.91 mm) |
| Recommended tap drill | 33/64 = .5156 in |
Alternative drills and % of thread
Any of these drills will tap 9/16-18. 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 |
|---|---|---|---|
| 33/64 | .5156 in | 13.10 | 65% |
Tips for tapping 9/16-18
- 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 (0.222" for 18 TPI).
- Thread engagement of 1–1.5 × the diameter (14.3–21.4 mm) is enough for most steel parts. Softer materials need more.