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3/8-24 tap drill size
Use a Q drill (.3320 in / 8.43 mm) to tap 3/8-24 UNF. That gives about 79% thread engagement. Closest metric drill: 8.4 mm.
3/8-24 thread specifications
| Thread | 3/8-24 UNF (fine) |
|---|---|
| Major (nominal) diameter | .3750 in / 9.53 mm |
| Threads per inch | 24 (pitch 1.058 mm) |
| Theoretical drill for 75% thread | .3344 in (8.49 mm) |
| Recommended tap drill | Q = .3320 in |
Alternative drills and % of thread
Any of these drills will tap 3/8-24. 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 3/8-24
- 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 (0.167" for 24 TPI).
- Thread engagement of 1–1.5 × the diameter (9.5–14.3 mm) is enough for most steel parts. Softer materials need more.