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#12-28 tap drill size
Use a #14 drill (.1820 in / 4.62 mm) to tap #12-28 UNF. That gives about 73% thread engagement. Closest metric drill: 4.6 mm.
#12-28 thread specifications
| Thread | #12-28 UNF (fine) |
|---|---|
| Major (nominal) diameter | .2160 in / 5.49 mm |
| Threads per inch | 28 (pitch 0.907 mm) |
| Theoretical drill for 75% thread | .1812 in (4.60 mm) |
| Recommended tap drill | #14 = .1820 in |
Alternative drills and % of thread
Any of these drills will tap #12-28. 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 |
|---|---|---|---|
| #16 | .1770 in | 4.50 | 84% |
| #15 | .1800 in | 4.57 | 78% |
| #14 | .1820 in | 4.62 | 73% |
| #13 | .1850 in | 4.70 | 67% |
| 3/16 | .1875 in | 4.76 | 61% |
| #12 | .1890 in | 4.80 | 58% |
| #11 | .1910 in | 4.85 | 54% |
Tips for tapping #12-28
- 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 (0.143" for 28 TPI).
- Thread engagement of 1–1.5 × the diameter (5.5–8.2 mm) is enough for most steel parts. Softer materials need more.