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#10-32 tap drill size
Use a #21 drill (.1590 in / 4.04 mm) to tap #10-32 UNF. That gives about 76% thread engagement. Closest metric drill: 4 mm.
#10-32 thread specifications
| Thread | #10-32 UNF (fine) |
|---|---|
| Major (nominal) diameter | .1900 in / 4.83 mm |
| Threads per inch | 32 (pitch 0.794 mm) |
| Theoretical drill for 75% thread | .1596 in (4.05 mm) |
| Recommended tap drill | #21 = .1590 in |
Alternative drills and % of thread
Any of these drills will tap #10-32. 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.
| Drill | Inch | mm | % thread |
|---|---|---|---|
| 5/32 | .1563 in | 3.97 | 83% |
| #22 | .1570 in | 3.99 | 81% |
| #21 | .1590 in | 4.04 | 76% |
| #20 | .1610 in | 4.09 | 71% |
| #19 | .1660 in | 4.22 | 59% |
| #18 | .1695 in | 4.30 | 51% |
Tips for tapping #10-32
- 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 (4.8–7.2 mm) is enough for most steel parts. Softer materials need more.