# 7/8-9 tap drill size

The tap drill for a 7/8-9 UNC thread is a 49/64 drill (0.7656 in, 19.45 mm). It leaves about 76 % thread. The closest metric drill is 19.4 mm, which leaves 77 % thread.

## Drill options for 7/8-9 UNC

| Drill | Inch | Thread % |
|---|---|---|
| 49/64 | 0.7656 | 76 % |
| 19.3 mm | 0.7598 | 80 % |
| 19.4 mm | 0.7638 | 77 % |
| 19.5 mm | 0.7677 | 74 % |

## 7/8-9 thread data

- Thread: 7/8-9 UNC, 9 TPI
- Major diameter: 0.8750 in (22.22 mm)
- Tap drill: 49/64 = 0.7656 in (19.45 mm)
- Thread engagement: 76 %

## Method

Thread % = TPI × (D − drill) / 0.01299 = 9 × 0.1094 / 0.01299 = 76 %.

## FAQ

**What size drill for a 7/8-9 tap?**
Use a 49/64 drill (0.7656 in, 19.45 mm). It leaves about 76 % thread.

**Can I use a metric drill for 7/8-9?**
Yes. A 19.4 mm drill gives about 77 % thread.

**How tight should a 7/8 grade 5 bolt be?**
About 429 ft-lb dry for 7/8-9 UNC.

Source page: https://machinistcharts.com/tap-drill/7-8-9/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)