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#5-40 tap drill size
Use a #38 drill (.1015 in / 2.58 mm) to tap #5-40 UNC. That gives about 72% thread engagement. Closest metric drill: 2.6 mm.
#5-40 thread specifications
| Thread | #5-40 UNC (coarse) |
|---|---|
| Major (nominal) diameter | .1250 in / 3.17 mm |
| Threads per inch | 40 (pitch 0.635 mm) |
| Theoretical drill for 75% thread | .1006 in (2.56 mm) |
| Recommended tap drill | #38 = .1015 in |
Alternative drills and % of thread
Any of these drills will tap #5-40. 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.
| Drill | Inch | mm | % thread |
|---|---|---|---|
| #40 | .0980 in | 2.49 | 83% |
| #39 | .0995 in | 2.53 | 79% |
| #38 | .1015 in | 2.58 | 72% |
| #37 | .1040 in | 2.64 | 65% |
| #36 | .1065 in | 2.71 | 57% |
Tips for tapping #5-40
- 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 (3.2–4.8 mm) is enough for most steel parts. Softer materials need more.