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5/8-18 tap drill size
Use a 37/64 drill (.5781 in / 14.68 mm) to tap 5/8-18 UNF. That gives about 65% thread engagement. Closest metric drill: 14.5 mm.
5/8-18 thread specifications
| Thread | 5/8-18 UNF (fine) |
|---|---|
| Major (nominal) diameter | .6250 in / 15.88 mm |
| Threads per inch | 18 (pitch 1.411 mm) |
| Theoretical drill for 75% thread | .5709 in (14.50 mm) |
| Recommended tap drill | 37/64 = .5781 in |
Alternative drills and % of thread
Any of these drills will tap 5/8-18. 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 |
|---|---|---|---|
| 37/64 | .5781 in | 14.68 | 65% |
Tips for tapping 5/8-18
- 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 (15.9–23.8 mm) is enough for most steel parts. Softer materials need more.