# M20 tap drill size

The tap drill for an M20 × 2.5 metric coarse thread is 17.5 mm (0.6890 in). It leaves about 77 % thread, the normal range for steel.

## Drill options for M20 × 2.5

| Drill | Inch | Thread % |
|---|---|---|
| 17.3 mm | 0.6811 | 83 % |
| 17.4 mm | 0.6850 | 80 % |
| 17.5 mm | 0.6890 | 77 % |
| 17.6 mm | 0.6929 | 74 % |
| 17.7 mm | 0.6969 | 71 % |

## M20 thread data

- Thread: M20 × 2.5 (coarse)
- Pitch: 2.5 mm
- Tap drill: 17.5 mm (0.6890 in)
- Thread engagement: 77 %
- Clearance hole (ISO 273): 21 close, 22 normal, 24 loose (mm)

## Method

Thread % = 76.98 × (D − drill) / P = 76.98 × 2.5 / 2.5 = 77 %. Aim for 65–75 % in steel, 75–85 % in aluminium, brass and plastics.

## FAQ

**What size drill for an M20 × 2.5 tap?**
Use a 17.5 mm drill. It leaves about 77 % thread engagement.

**What is 17.5 mm in inches?**
17.5 mm is 0.6890 inch.

**What size clearance hole for M20?**
ISO 273 gives 21 mm for a close fit, 22 mm normal and 24 mm loose.

**How tight should an M20 bolt be?**
About 441 N·m for class 8.8 with dry threads.

Source page: https://machinistcharts.com/tap-drill/m20/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)