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3/4-10 tap drill size
Use a 21/32 drill (.6563 in / 16.67 mm) to tap 3/4-10 UNC. That gives about 72% thread engagement. Closest metric drill: 16.5 mm.
3/4-10 thread specifications
| Thread | 3/4-10 UNC (coarse) |
|---|---|
| Major (nominal) diameter | .7500 in / 19.05 mm |
| Threads per inch | 10 (pitch 2.54 mm) |
| Theoretical drill for 75% thread | .6526 in (16.58 mm) |
| Recommended tap drill | 21/32 = .6563 in |
Alternative drills and % of thread
Any of these drills will tap 3/4-10. 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 3/4-10
- 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 (0.400" for 10 TPI).
- Thread engagement of 1–1.5 × the diameter (19.1–28.6 mm) is enough for most steel parts. Softer materials need more.