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7/8-9 tap drill size
Use a 49/64 drill (.7656 in / 19.45 mm) to tap 7/8-9 UNC. That gives about 76% thread engagement. Closest metric drill: 19.5 mm.
7/8-9 thread specifications
| Thread | 7/8-9 UNC (coarse) |
|---|---|
| Major (nominal) diameter | .8750 in / 22.23 mm |
| Threads per inch | 9 (pitch 2.822 mm) |
| Theoretical drill for 75% thread | .7668 in (19.48 mm) |
| Recommended tap drill | 49/64 = .7656 in |
Alternative drills and % of thread
Any of these drills will tap 7/8-9. 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 7/8-9
- 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 (0.444" for 9 TPI).
- Thread engagement of 1–1.5 × the diameter (22.2–33.3 mm) is enough for most steel parts. Softer materials need more.