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3/8-16 tap drill size
Use a 5/16 drill (.3125 in / 7.94 mm) to tap 3/8-16 UNC. That gives about 77% thread engagement. Closest metric drill: 7.9 mm.
3/8-16 thread specifications
| Thread | 3/8-16 UNC (coarse) |
|---|---|
| Major (nominal) diameter | .3750 in / 9.53 mm |
| Threads per inch | 16 (pitch 1.587 mm) |
| Theoretical drill for 75% thread | .3141 in (7.98 mm) |
| Recommended tap drill | 5/16 = .3125 in |
Alternative drills and % of thread
Any of these drills will tap 3/8-16. 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 |
|---|---|---|---|
| 5/16 | .3125 in | 7.94 | 77% |
| O | .3160 in | 8.03 | 73% |
| P | .3230 in | 8.20 | 64% |
| 21/64 | .3281 in | 8.33 | 58% |
| Q | .3320 in | 8.43 | 53% |
Tips for tapping 3/8-16
- 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 (0.250" for 16 TPI).
- Thread engagement of 1–1.5 × the diameter (9.5–14.3 mm) is enough for most steel parts. Softer materials need more.