3-1/4-8 tap drill size
Use a 3-1/8 drill (3.1250 in) for a 3-1/4-8 cutting tap at 75% thread.
| Tap type | Drill | Decimal (in) |
|---|---|---|
| Cutting tap, 75% thread | 3-1/8 | 3.1250 |
| Cutting tap, 50% thread (hard material) | 3-3/16 | 3.1875 |
| Roll / form tap, 65% thread | 3-3/16 | 3.1875 |
- Pitch
- 3.175 mm (8 TPI)
- Major Ø
- 3.2500
- Pitch Ø
- 3.1688
- Minor Ø
- 3.1147
Common questions
- What size drill for a 3-1/4-8 tap?
- Use a 3-1/8 drill (3.1250 in) for a 3-1/4-8 cutting tap at 75% thread.
- What drill for a 3-1/4-8 roll (form) tap?
- About 3.1948 in; the nearest standard drill is 3-3/16. Check your tap maker's table.
Other 8UN 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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