# M39 tap drill size

The tap drill for an M39 × 4 metric coarse thread is 35 mm (1.3780 in). It leaves about 77 % thread, the normal range for steel.

## Drill options for M39 × 4

| Drill | Inch | Thread % |
|---|---|---|
| 34.8 mm | 1.3701 | 81 % |
| 34.9 mm | 1.3740 | 79 % |
| 35 mm | 1.3780 | 77 % |
| 35.1 mm | 1.3819 | 75 % |
| 35.2 mm | 1.3858 | 73 % |

## M39 thread data

- Thread: M39 × 4 (coarse)
- Pitch: 4 mm
- Tap drill: 35 mm (1.3780 in)
- Thread engagement: 77 %
- Clearance hole (ISO 273): 40 close, 42 normal, 45 loose (mm)

## Method

Thread % = 76.98 × (D − drill) / P = 76.98 × 4 / 4 = 77 %. Aim for 65–75 % in steel, 75–85 % in aluminium, brass and plastics.

## FAQ

**What size drill for an M39 × 4 tap?**
Use a 35 mm drill. It leaves about 77 % thread engagement.

**What is 35 mm in inches?**
35 mm is 1.3780 inch.

**What size clearance hole for M39?**
ISO 273 gives 40 mm for a close fit, 42 mm normal and 45 mm loose.

Source page: https://machinistcharts.com/tap-drill/m39/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)