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#4-40 tap drill size
Use a #43 drill (.0890 in / 2.26 mm) to tap #4-40 UNC. That gives about 71% thread engagement. Closest metric drill: 2.3 mm.
#4-40 thread specifications
| Thread | #4-40 UNC (coarse) |
|---|---|
| Major (nominal) diameter | .1120 in / 2.85 mm |
| Threads per inch | 40 (pitch 0.635 mm) |
| Theoretical drill for 75% thread | .0876 in (2.23 mm) |
| Recommended tap drill | #43 = .0890 in |
Alternative drills and % of thread
Any of these drills will tap #4-40. 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 #4-40
- 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 (0.100" for 40 TPI).
- Thread engagement of 1–1.5 × the diameter (2.8–4.3 mm) is enough for most steel parts. Softer materials need more.