# Letter R drill size

A letter R drill is 0.3390 in (8.61 mm). It sits between letter Q and letter S in the letter series; the nearest metric drill is 8.6 mm.

## Letter R drill data

- Series: Letter drill
- Decimal: 0.3390 in
- Metric: 8.61 mm (8.6106 mm exact)
- Nearest metric drill: 8.6 mm (−0.011 mm)
- Nearest fractional: 11/32" (0.3438 in)

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 8.6 mm | 0.3386 | −0.011 mm |
| 8.7 mm | 0.3425 | +0.089 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| #1 | Number | 0.2280 | 5.79 |
| 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 |
| Letter S | Letter | 0.3480 | 8.84 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| M10 × 1.5 | 8.5 mm | 71 |
| 3/8-24 UNF | Q | 67 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| 5/16" bolt | 0.3125" | 0.0265" | 0.0133" | – |
| M8 bolt | 8 mm | 0.61 mm | 0.31 mm | Close fit (close 8.4, normal 9 mm) |


## FAQ

**What size is a letter R drill?**
A letter R drill is 0.3390 in, which is 8.61 mm (8.6106 mm exactly).

**What is the metric equivalent of a letter R drill?**
The nearest metric drill is 8.6 mm, 0.011 mm smaller. The other close size is 8.7 mm.

**What thread can I tap with a letter R drill?**
It leaves about 71 % thread in M10 × 1.5 and 67 % in 3/8-24 UNF.

**Which fractional drill is closest to letter R?**
11/32 in (0.3438 in) is the closest, 0.0047 in larger.

**What does a letter R drill clear?**
It clears a M8 bolt with 0.61 mm total clearance (0.31 mm per side); for metric bolts: close fit.

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