4-40 tap drill size
Use a #43 drill (0.0890 in) for a 4-40 cutting tap at 75% thread.
| Tap type | Drill | Decimal (in) |
|---|---|---|
| Cutting tap, 75% thread | #43 | 0.0890 |
| Cutting tap, 50% thread (hard material) | #41 | 0.0960 |
| Roll / form tap, 65% thread | #38 | 0.1015 |
| Helicoil / STI insert (#4-40×1D) | #31 | 0.1200 |
- Pitch
- 0.635 mm (40 TPI)
- Major Ø
- 0.1120
- Pitch Ø
- 0.0958
- Minor Ø
- 0.0813
4-40 tapping speeds and feeds
Feed = pitch (0.0250 in per rev). Starting points for a general-purpose HSS cut tap:
- Aluminum 6061 / 7075: 2,390 RPM · 59.8 IPM
- Free-machining / mild steel: 680 RPM · 17.0 IPM
- 304 / 316 stainless: 340 RPM · 8.50 IPM
4-40 tap speeds and feeds: all materials, form taps and G84 code · #43 drill size and speeds
Common questions
- What size drill for a 4-40 tap?
- Use a #43 drill (0.0890 in) for a 4-40 cutting tap at 75% thread.
- What drill for a 4-40 roll (form) tap?
- About 0.1010 in; the nearest standard drill is #38. Check your tap maker's table.
- What drill for a 4-40 Helicoil?
- Drill #31 (0.1200 in), then tap with the 4-40 STI tap.
Other UNC sizes
Drill sizes from Machinery's Handbook and ASME B1.1/B1.13M; roll tap sizes from the form-tap formula. Tap and insert makers' tables are authoritative.
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