# 3/8-24 tap drill size

The tap drill for a 3/8-24 UNF thread is a Q drill (0.3320 in, 8.43 mm). It leaves about 79 % thread. The closest metric drill is 8.4 mm, which leaves 82 % thread.

## Drill options for 3/8-24 UNF

| Drill | Inch | Thread % |
|---|---|---|
| Q | 0.3320 | 79 % |
| 8.3 mm | 0.3268 | 89 % |
| 8.4 mm | 0.3307 | 82 % |
| 8.5 mm | 0.3347 | 75 % |

## 3/8-24 thread data

- Thread: 3/8-24 UNF, 24 TPI
- Major diameter: 0.3750 in (9.52 mm)
- Tap drill: Q = 0.3320 in (8.43 mm)
- Thread engagement: 79 %

## Method

Thread % = TPI × (D − drill) / 0.01299 = 24 × 0.0430 / 0.01299 = 79 %.

## FAQ

**What size drill for a 3/8-24 tap?**
Use a Q drill (0.3320 in, 8.43 mm). It leaves about 79 % thread.

**Can I use a metric drill for 3/8-24?**
Yes. A 8.4 mm drill gives about 82 % thread.

**How tight should a 3/8 grade 5 bolt be?**
About 35 ft-lb dry for 3/8-24 UNF.

Source page: https://machinistcharts.com/tap-drill/3-8-24/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)