A letter o drill measures 0.3160 inch (8.026 mm). The closest fractional drill is 5/16" (0.3125") and the closest metric drill is 8.0 mm (0.3150").
What are the letter o drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| O | letter | 0.3160 | 8.026 | 0 |
| 8.0 mm | metric | 0.3150 | 8.000 | -0.0010 |
| 8.1 mm | metric | 0.3189 | 8.100 | +0.0029 |
Drill sets that include this size
Number and letter sizes come in indexed sets; buy the set rather than single drills.
What speed should a letter o drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 2,418 | 0.0063 |
| 360 free-machining brass | 1,813 | 0.0057 |
| 1018 cold rolled steel | 967 | 0.0047 |
| 304 stainless steel | 484 | 0.0032 |
Starting points from conservative HSS drilling speeds. Small drills often call for more RPM than a benchtop drill press offers; run at the top speed and feed gently. Use cutting oil in steel and stainless, and clamp the work: never hold it by hand.
Cutting fluid for drilling
Frequently asked questions
What is a letter o drill in inches?
0.3160 inch, which is 8.026 mm. Letter drills run from A (0.234 inch) to Z (0.413 inch), continuing upward from the number drills. If you do not have this exact size, 8.0 mm is within 0.0010 inch.
What tap uses a letter o drill?
No common UNC or UNF thread uses it as its standard 75% tap drill, but it often serves as an alternative drill for a slightly different percentage of thread, or as a clearance or pilot size. The tap drill chart lists every standard pairing.