---
type: "drill"
designation: "Y"
series: "letter"
series_name: "Letter Drill Sizes"
diameter_in: 0.404
diameter_mm: 10.2616
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/letter/y
---

# Y Drill Bit Size — 0.4040 in / 10.262 mm

A Y drill bit is 0.4040 in (10.262 mm) in diameter. It belongs to the letter series of standard twist drills. It is the recommended tap drill for 7/16"-28 UNEF.

- **Designation**: Y
- **Series**: Letter
- **Diameter**: 0.4040 in (10.262 mm)

## Y Drill Diameter

| Property | Value |
| --- | --- |
| Designation | Y |
| Series | Letter |
| Diameter (in) | 0.4040 |
| Diameter (mm) | 10.262 |

## Y Drill Equivalents in the Other Size Series

The nearest standard drill to Y in each of the other three series. Nothing here is an exact match unless the difference column reads 0.0000 in.

| Series | Nearest drill | Diameter (in) | Diameter (mm) | Difference |
| --- | --- | --- | --- | --- |
| Number | [#1](https://shopmath.org/drills/number/1.md) | 0.2280 | 5.791 | -0.1760 in |
| Fractional | [13/32](https://shopmath.org/drills/fractional/13-32.md) | 0.4063 | 10.319 | +0.0022 in |
| Metric | [10.3 mm](https://shopmath.org/drills/metric/10-3mm.md) | 0.4055 | 10.300 | +0.0015 in |

## Threads Tapped With a Y Drill

| Thread | Pitch | Thread engagement (%) |
| --- | --- | --- |
| [7/16"-28 UNEF](https://shopmath.org/threads/unef/7-16-28.md) | 28 TPI | 72.2 |

_Engagement is what the real drill leaves with a cut tap, not the requested target._

## Sources and Method for the Y Drill Size

### Drill index

- **Source**: Number/letter: bd_warehouse drill_sizes.csv (Apache-2.0) cross-checked against Wikipedia Drill bit sizes (CC BY-SA 4.0). Fractional: computed 1/64 in steps per ANSI B94.11M. Metric: computed BS 328 increment pattern.
- **Method**: compiled
- **Verified against**: Wikipedia Drill bit sizes (CC BY-SA 4.0); 31/31 checks passed

### Assumptions

- Drill diameters are nominal. A worn or run-out drill cuts oversize; measure the hole, not the shank, when the fit matters.
- Dataset scope: Number sizes #61-#80 are stored but tagged demandTier 'extended' — query-space-map.md Appendix A found the tail thinning in the 60s. Sizes below #80 and metric sizes above 25.0 mm excluded.

**Data last verified**: August 1, 2026

## Common Questions About the Y Drill Bit

### What size is a Y drill bit size?

A Y drill bit is 0.4040 in (10.262 mm) in diameter.

### What is the metric equivalent of a Y drill bit?

The closest metric drill to Y (0.4040 in (10.262 mm)) is 10.3 mm at 0.4055 in (10.300 mm) — a difference of 0.0015 in.

### What fraction is a Y drill bit?

Y is 0.4040 in (10.262 mm), which is not itself a fractional size. The closest fractional drill is 13/32 at 0.4063 in (10.319 mm).

### What thread does a Y drill bit tap?

A Y drill is the recommended tap drill for 7/16"-28 UNEF. That leaves the thread engagement shown in the table above with a cut tap.

## Related pages

- Next size down (any series): [10.2 mm](https://shopmath.org/drills/metric/10-2mm.md) — 0.4016 in (10.200 mm)
- Next size up (any series): [10.3 mm](https://shopmath.org/drills/metric/10-3mm.md) — 0.4055 in (10.300 mm)
- Previous in the letter series: [X](https://shopmath.org/drills/letter/x.md)
- Next in the letter series: [Z](https://shopmath.org/drills/letter/z.md)
- All letter sizes: [Letter drill sizes](https://shopmath.org/drills/letter.md)
- All drill sizes: [Drill bit size reference](https://shopmath.org/drills.md)
- Complete index by diameter: [Drill size chart](https://shopmath.org/charts/drill-sizes.md)
- Tap drills by thread: [Tap drill chart](https://shopmath.org/charts/tap-drill.md)

---

*Source: [ShopMath](https://shopmath.org/drills/letter/y) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*