1/4-20 tap drill size
Use a #7 drill (0.2010 in) for a 1/4-20 cutting tap at 75% thread.
| Tap type | Drill | Decimal (in) |
|---|---|---|
| Cutting tap, 75% thread | #7 | 0.2010 |
| Cutting tap, 50% thread (hard material) | 7/32" | 0.2188 |
| Roll / form tap, 65% thread | #1 | 0.2280 |
| Helicoil / STI insert (1/4-20×1D) | F | 0.2570 |
- Pitch
- 1.27 mm (20 TPI)
- Major Ø
- 0.2500
- Pitch Ø
- 0.2175
- Minor Ø
- 0.1887
1/4-20 tapping speeds and feeds
Feed = pitch (0.0500 in per rev). Starting points for a general-purpose HSS cut tap:
- Aluminum 6061 / 7075: 1,070 RPM · 53.5 IPM
- Free-machining / mild steel: 310 RPM · 15.5 IPM
- 304 / 316 stainless: 150 RPM · 7.50 IPM
1/4-20 tap speeds and feeds: all materials, form taps and G84 code · #7 drill size and speeds
Common questions
- What size drill for a 1/4-20 tap?
- Use a #7 drill (0.2010 in) for a 1/4-20 cutting tap at 75% thread.
- What drill for a 1/4-20 roll (form) tap?
- About 0.2279 in; the nearest standard drill is #1. Check your tap maker's table.
- What drill for a 1/4-20 Helicoil?
- Drill F (0.2570 in), then tap with the 1/4-20 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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