# Letter Y drill size

A letter Y drill is 0.4040 in (10.26 mm). It sits between letter X and letter Z in the letter series; the nearest metric drill is 10.3 mm.

## Letter Y drill data

- Series: Letter drill
- Decimal: 0.4040 in
- Metric: 10.26 mm (10.2616 mm exact)
- Nearest metric drill: 10.3 mm (+0.038 mm)
- Nearest fractional: 13/32" (0.4062 in)

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 10.2 mm | 0.4016 | −0.062 mm |
| 10.3 mm | 0.4055 | +0.038 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| #1 | Number | 0.2280 | 5.79 |
| 25/64" | Fractional | 0.3906 | 9.92 |
| Letter X | Letter | 0.3970 | 10.08 |
| Letter Y | Letter | 0.4040 | 10.26 |
| 13/32" | Fractional | 0.4062 | 10.32 |
| Letter Z | Letter | 0.4130 | 10.49 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| M12 × 1.75 | 10.2 mm | 76 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| 3/8" bolt | 0.3750" | 0.0290" | 0.0145" | – |
| M10 bolt | 10 mm | 0.26 mm | 0.13 mm | Tighter than close fit (10.5 mm) |


## FAQ

**What size is a letter Y drill?**
A letter Y drill is 0.4040 in, which is 10.26 mm (10.2616 mm exactly).

**What is the metric equivalent of a letter Y drill?**
The nearest metric drill is 10.3 mm, 0.038 mm larger. The other close size is 10.2 mm.

**What thread can I tap with a letter Y drill?**
It leaves about 76 % thread in M12 × 1.75.

**Which fractional drill is closest to letter Y?**
13/32 in (0.4062 in) is the closest, 0.0022 in larger.

**What does a letter Y drill clear?**
It clears a M10 bolt with 0.26 mm total clearance (0.13 mm per side); for metric bolts: tighter than close fit.

Source page: https://machinistcharts.com/drill-size/letter-y/
Updated: 2026-10-06
Publisher: MachinistCharts (https://machinistcharts.com)