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#6-40 tap drill size
Use a #33 drill (.1130 in / 2.87 mm) to tap #6-40 UNF. That gives about 77% thread engagement. Closest metric drill: 2.9 mm.
#6-40 thread specifications
| Thread | #6-40 UNF (fine) |
|---|---|
| Major (nominal) diameter | .1380 in / 3.50 mm |
| Threads per inch | 40 (pitch 0.635 mm) |
| Theoretical drill for 75% thread | .1136 in (2.89 mm) |
| Recommended tap drill | #33 = .1130 in |
Alternative drills and % of thread
Any of these drills will tap #6-40. 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 #6-40
- 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 (3.5–5.3 mm) is enough for most steel parts. Softer materials need more.