# Letter Q drill size

A letter Q drill is 0.3320 in (8.43 mm). It is the standard tap drill for 3/8-24 UNF; the nearest metric drill is 8.4 mm.

## Letter Q drill data

- Series: Letter drill
- Decimal: 0.3320 in
- Metric: 8.43 mm (8.4328 mm exact)
- Nearest metric drill: 8.4 mm (−0.033 mm)
- Nearest fractional: 21/64" (0.3281 in)
- Standard tap drill for: 3/8-24 UNF

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 8.4 mm | 0.3307 | −0.033 mm |
| 8.5 mm | 0.3346 | +0.067 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| #1 | Number | 0.2280 | 5.79 |
| Letter P | Letter | 0.3230 | 8.20 |
| 21/64" | Fractional | 0.3281 | 8.33 |
| Letter Q | Letter | 0.3320 | 8.43 |
| Letter R | Letter | 0.3390 | 8.61 |
| 11/32" | Fractional | 0.3438 | 8.73 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| 3/8-24 UNF | Q | 79 |
| M10 × 1.5 | 8.5 mm | 80 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| 5/16" bolt | 0.3125" | 0.0195" | 0.0098" | – |
| M8 bolt | 8 mm | 0.43 mm | 0.22 mm | Close fit (close 8.4, normal 9 mm) |


## FAQ

**What size is a letter Q drill?**
A letter Q drill is 0.3320 in, which is 8.43 mm (8.4328 mm exactly).

**What is the metric equivalent of a letter Q drill?**
The nearest metric drill is 8.4 mm, 0.033 mm smaller. The other close size is 8.5 mm.

**What tap uses a letter Q drill?**
A letter Q drill is the standard tap drill for 3/8-24 UNF (about 79 % thread).

**Which fractional drill is closest to letter Q?**
21/64 in (0.3281 in) is the closest, 0.0039 in smaller.

**What does a letter Q drill clear?**
It clears a M8 bolt with 0.43 mm total clearance (0.22 mm per side); for metric bolts: close fit.

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