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#10-24 tap drill size
Use a #25 drill (.1495 in / 3.80 mm) to tap #10-24 UNC. That gives about 75% thread engagement. Closest metric drill: 3.8 mm.
#10-24 thread specifications
| Thread | #10-24 UNC (coarse) |
|---|---|
| Major (nominal) diameter | .1900 in / 4.83 mm |
| Threads per inch | 24 (pitch 1.058 mm) |
| Theoretical drill for 75% thread | .1494 in (3.79 mm) |
| Recommended tap drill | #25 = .1495 in |
Alternative drills and % of thread
Any of these drills will tap #10-24. 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 |
|---|---|---|---|
| #27 | .1440 in | 3.66 | 85% |
| #26 | .1470 in | 3.73 | 79% |
| #25 | .1495 in | 3.80 | 75% |
| #24 | .1520 in | 3.86 | 70% |
| #23 | .1540 in | 3.91 | 67% |
| 5/32 | .1563 in | 3.97 | 62% |
| #22 | .1570 in | 3.99 | 61% |
| #21 | .1590 in | 4.04 | 57% |
| #20 | .1610 in | 4.09 | 54% |
Tips for tapping #10-24
- 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 (0.167" for 24 TPI).
- Thread engagement of 1–1.5 × the diameter (4.8–7.2 mm) is enough for most steel parts. Softer materials need more.