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5/16-18 tap drill size
Use a F drill (.2570 in / 6.53 mm) to tap 5/16-18 UNC. That gives about 77% thread engagement. Closest metric drill: 6.5 mm.
5/16-18 thread specifications
| Thread | 5/16-18 UNC (coarse) |
|---|---|
| Major (nominal) diameter | .3125 in / 7.94 mm |
| Threads per inch | 18 (pitch 1.411 mm) |
| Theoretical drill for 75% thread | .2584 in (6.56 mm) |
| Recommended tap drill | F = .2570 in |
Alternative drills and % of thread
Any of these drills will tap 5/16-18. 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 |
|---|---|---|---|
| F | .2570 in | 6.53 | 77% |
| G | .2610 in | 6.63 | 71% |
| 17/64 | .2656 in | 6.75 | 65% |
| H | .2660 in | 6.76 | 64% |
| I | .2720 in | 6.91 | 56% |
Tips for tapping 5/16-18
- 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 (7.9–11.9 mm) is enough for most steel parts. Softer materials need more.