# #51 drill size

A #51 drill is 0.0670 in (1.70 mm). It sits between #52 and #50 in the number series; the nearest metric drill is 1.7 mm.

## #51 drill data

- Series: Number drill
- Decimal: 0.0670 in
- Metric: 1.70 mm (1.7018 mm exact)
- Nearest metric drill: 1.7 mm (−0.002 mm)
- Nearest fractional: 1/16" (0.0625 in)

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 1.7 mm | 0.0669 | −0.002 mm |
| 1.75 mm | 0.0689 | +0.048 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| 1/16" | Fractional | 0.0625 | 1.59 |
| #52 | Number | 0.0635 | 1.61 |
| #51 | Number | 0.0670 | 1.70 |
| #50 | Number | 0.0700 | 1.78 |
| 5/64" | Fractional | 0.0781 | 1.98 |
| Letter A | Letter | 0.2340 | 5.94 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| #2-56 UNC | #50 | 82 |
| M2 × 0.4 | 1.6 mm | 57 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| M1.6 bolt | 1.6 mm | 0.10 mm | 0.05 mm | Close fit (close 1.7, normal 1.8 mm) |


## FAQ

**What size is a #51 drill?**
A #51 drill is 0.0670 in, which is 1.70 mm (1.7018 mm exactly).

**What is the metric equivalent of a #51 drill?**
The nearest metric drill is 1.7 mm, 0.002 mm smaller. The other close size is 1.75 mm.

**What thread can I tap with a #51 drill?**
It leaves about 82 % thread in #2-56 UNC and 57 % in M2 × 0.4.

**Which fractional drill is closest to #51?**
1/16 in (0.0625 in) is the closest, 0.0045 in smaller.

**What does a #51 drill clear?**
It clears a M1.6 bolt with 0.10 mm total clearance (0.05 mm per side); for metric bolts: close fit.

Source page: https://machinistcharts.com/drill-size/number-51/
Updated: 2026-10-06
Publisher: MachinistCharts (https://machinistcharts.com)