3/8-24 tap drill size
Use a Q drill (0.3320 in) for a 3/8-24 cutting tap at 75% thread.
| Tap type | Drill | Decimal (in) |
|---|---|---|
| Cutting tap, 75% thread | Q | 0.3320 |
| Cutting tap, 50% thread (hard material) | S | 0.3480 |
| Roll / form tap, 65% thread | T | 0.3580 |
| Helicoil / STI insert (3/8-24×1D) | 27/64" | 0.4219 |
- Pitch
- 1.0583 mm (24 TPI)
- Major Ø
- 0.3750
- Pitch Ø
- 0.3479
- Minor Ø
- 0.3239
3/8-24 tapping speeds and feeds
Feed = pitch (0.0417 in per rev). Starting points for a general-purpose HSS cut tap:
- Aluminum 6061 / 7075: 710 RPM · 29.6 IPM
- Free-machining / mild steel: 200 RPM · 8.33 IPM
- 304 / 316 stainless: 100 RPM · 4.17 IPM
3/8-24 tap speeds and feeds: all materials, form taps and G84 code · Q drill size and speeds
Common questions
- What size drill for a 3/8-24 tap?
- Use a Q drill (0.3320 in) for a 3/8-24 cutting tap at 75% thread.
- What drill for a 3/8-24 roll (form) tap?
- About 0.3566 in; the nearest standard drill is T. Check your tap maker's table.
- What drill for a 3/8-24 Helicoil?
- Drill 27/64" (0.4219 in), then tap with the 3/8-24 STI tap.
Other UNF 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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