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#3-56 tap drill size
Use a #45 drill (.0820 in / 2.08 mm) to tap #3-56 UNF. That gives about 73% thread engagement. Closest metric drill: 2.1 mm.
#3-56 thread specifications
| Thread | #3-56 UNF (fine) |
|---|---|
| Major (nominal) diameter | .0990 in / 2.52 mm |
| Threads per inch | 56 (pitch 0.454 mm) |
| Theoretical drill for 75% thread | .0816 in (2.07 mm) |
| Recommended tap drill | #45 = .0820 in |
Alternative drills and % of thread
Any of these drills will tap #3-56. 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 #3-56
- 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 (0.071" for 56 TPI).
- Thread engagement of 1–1.5 × the diameter (2.5–3.8 mm) is enough for most steel parts. Softer materials need more.