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#4-48 tap drill size
Use a #42 drill (.0935 in / 2.38 mm) to tap #4-48 UNF. That gives about 68% thread engagement. Closest metric drill: 2.4 mm.
#4-48 thread specifications
| Thread | #4-48 UNF (fine) |
|---|---|
| Major (nominal) diameter | .1120 in / 2.85 mm |
| Threads per inch | 48 (pitch 0.529 mm) |
| Theoretical drill for 75% thread | .0917 in (2.33 mm) |
| Recommended tap drill | #42 = .0935 in |
Alternative drills and % of thread
Any of these drills will tap #4-48. 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 |
|---|---|---|---|
| #43 | .0890 in | 2.26 | 85% |
| #42 | .0935 in | 2.38 | 68% |
| 3/32 | .0938 in | 2.38 | 67% |
| #41 | .0960 in | 2.44 | 59% |
| #40 | .0980 in | 2.49 | 52% |
Tips for tapping #4-48
- 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 (0.083" for 48 TPI).
- Thread engagement of 1–1.5 × the diameter (2.8–4.3 mm) is enough for most steel parts. Softer materials need more.