A letter y drill measures 0.4040 inch (10.262 mm). The closest fractional drill is 13/32" (0.4063") and the closest metric drill is 10.3 mm (0.4055").
What are the letter y drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| Y | letter | 0.4040 | 10.262 | 0 |
| 10.2 mm | metric | 0.4016 | 10.200 | -0.0024 |
| 10.3 mm | metric | 0.4055 | 10.300 | +0.0015 |
| 13/32" | fraction | 0.4063 | 10.319 | +0.0023 |
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 y drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 1,891 | 0.0081 |
| 360 free-machining brass | 1,418 | 0.0073 |
| 1018 cold rolled steel | 756 | 0.0061 |
| 304 stainless steel | 378 | 0.0040 |
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 y drill in inches?
0.4040 inch, which is 10.262 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, 10.3 mm is within 0.0015 inch.
What tap uses a letter y 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.