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#8-32 tap drill size
Use a #29 drill (.1360 in / 3.45 mm) to tap #8-32 UNC. That gives about 69% thread engagement. Closest metric drill: 3.5 mm.
#8-32 thread specifications
| Thread | #8-32 UNC (coarse) |
|---|---|
| Major (nominal) diameter | .1640 in / 4.17 mm |
| Threads per inch | 32 (pitch 0.794 mm) |
| Theoretical drill for 75% thread | .1336 in (3.39 mm) |
| Recommended tap drill | #29 = .1360 in |
Alternative drills and % of thread
Any of these drills will tap #8-32. 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 #8-32
- 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 (0.125" for 32 TPI).
- Thread engagement of 1–1.5 × the diameter (4.2–6.2 mm) is enough for most steel parts. Softer materials need more.