#4-40 tap drill size.
For a #4-40 tap, use a #43 drill (0.0890 in, 2.26 mm). This gives about 71% thread engagement. For a clearance hole, use a #32 drill (0.1160 in).
#4-40 at a glance
| Item | Value |
|---|---|
| Thread | #4-40 (Unified UNC coarse) |
| Major diameter | 0.1120 in (2.84 mm) |
| Threads per inch | 40 (pitch 0.0250 in, 0.635 mm) |
| Tap drill | #43 drill: 0.0890 in (2.26 mm) |
| Thread engagement | about 71% |
| Clearance drill | #32 drill: 0.1160 in (2.95 mm) |
How the tap drill is chosen
- Major diameter: 0.1120 in. Threads per inch: 40.
- For 75% thread: 0.1120 − 0.9743 ÷ 40 = 0.0876 in.
- Use the standard drill nearest that size: #43 drill (0.0890 in), about 71% thread.
Drill the tap hole, deburr it, then tap with cutting fluid. In blind holes, drill deeper than the thread length: allow about 2–3 thread pitches below the last full thread.
Common questions
What size drill for a #4-40 tap?
For a #4-40 tap, use a #43 drill (0.0890 in, 2.26 mm).
What is the clearance hole for #4-40?
Use a #32 drill (0.1160 in, 2.95 mm). A clearance hole lets the screw pass through without threading.
What thread percentage does this drill give?
About 71%. Machinery's Handbook defines percent of thread as 76.98 × TPI × (major diameter − drill diameter). 75% is the common target: more engagement adds little strength but makes tapping much harder.
Can I use a slightly larger drill?
Yes, in hard materials such as stainless steel or titanium. A larger drill gives a lower thread percentage, which makes tapping easier and reduces tap breakage. Use the formula above to check the result.
Sources
- Tap drill sizes: Machinery's Handbook, 31st ed. — tap drill sizes for 75% thread.
- Drill diameters: ANSI B94.11M — twist drill sizes (number, letter and fractional series).
- Thread dimensions: ASME B1.1 — Unified inch screw threads.
Other tap drill sizes
#6-32 · #8-32 · #10-24 · #10-32 · 1/4-20 · 1/4-28 · 5/16-18 · 3/8-16 · 3/8-24 · 1/2-13
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