---
type: "drill"
designation: "4.2 mm"
series: "metric"
series_name: "Metric Drill Sizes"
diameter_in: 0.165354
diameter_mm: 4.2
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/metric/4-2mm
---

# 4.2 mm Drill Bit Size — 0.1654 in / 4.200 mm

A 4.2 mm drill bit is 0.1654 in (4.200 mm) in diameter. It belongs to the metric series of standard twist drills. It is the recommended tap drill for M5×0.8.

- **Designation**: 4.2 mm
- **Series**: Metric
- **Diameter**: 0.1654 in (4.200 mm)

## 4.2 mm Drill Diameter

| Property | Value |
| --- | --- |
| Designation | 4.2 mm |
| Series | Metric |
| Diameter (in) | 0.1654 |
| Diameter (mm) | 4.200 |

## 4.2 mm Drill Equivalents in the Other Size Series

The nearest standard drill to 4.2 mm 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 | [#19](https://shopmath.org/drills/number/19.md) | 0.1660 | 4.216 | +0.0006 in |
| Letter | [A](https://shopmath.org/drills/letter/a.md) | 0.2340 | 5.944 | +0.0686 in |
| Fractional | [11/64](https://shopmath.org/drills/fractional/11-64.md) | 0.1719 | 4.366 | +0.0065 in |

## Threads Tapped With a 4.2 mm Drill

| Thread | Pitch | Thread engagement (%) |
| --- | --- | --- |
| [M5×0.8](https://shopmath.org/threads/metric-coarse/m5x0.8.md) | 0.800 mm | 77.0 |

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

## Sources and Method for the 4.2 mm 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**: computed
- **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 4.2 mm Drill Bit

### What size is a 4.2 mm drill bit size?

A 4.2 mm drill bit is 0.1654 in (4.200 mm) in diameter.

### What fraction is a 4.2 mm drill bit?

4.2 mm is 0.1654 in (4.200 mm), which is not itself a fractional size. The closest fractional drill is 11/64 at 0.1719 in (4.366 mm).

### What thread does a 4.2 mm drill bit tap?

A 4.2 mm drill is the recommended tap drill for M5×0.8. That leaves the thread engagement shown in the table above with a cut tap.

## Related pages

- Next size down (any series): [4.1 mm](https://shopmath.org/drills/metric/4-1mm.md) — 0.1614 in (4.100 mm)
- Next size up (any series): [#19](https://shopmath.org/drills/number/19.md) — 0.1660 in (4.216 mm)
- Previous in the metric series: [4.1 mm](https://shopmath.org/drills/metric/4-1mm.md)
- Next in the metric series: [4.3 mm](https://shopmath.org/drills/metric/4-3mm.md)
- All metric sizes: [Metric drill sizes](https://shopmath.org/drills/metric.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/metric/4-2mm) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*