# 3/8-16 tap drill size

URL: https://www.clove3d.com/tools/tap-drill/3-8-16/

Tap drill for 3/8-16 UNC: 5/16" (0.3125 in / 7.94 mm), about 77% thread.

## Thread specs

- Series: UNC
- Major diameter: 0.3750 in
- Pitch: 16 TPI
- Basic minor diameter: 0.3073 in

## Every drill that works (50-85% thread)

| Drill | % thread | in | mm | Good for |
|---|---|---|---|---|
| Q | 53% | 0.3320 | 8.43 | deep holes only |
| 8.4 mm | 55% | 0.3307 | 8.40 | deep holes only |
| 21/64" | 58% | 0.3281 | 8.33 | deep holes only |
| 8.3 mm | 59% | 0.3268 | 8.30 | deep holes only |
| P | 64% | 0.3230 | 8.20 | hard or tough metals |
| 8.2 mm | 64% | 0.3228 | 8.20 | hard or tough metals |
| 8.1 mm | 69% | 0.3189 | 8.10 | hard or tough metals |
| O | 73% | 0.3160 | 8.03 | general use |
| 8.0 mm | 74% | 0.3150 | 8.00 | general use |
| 5/16" (standard) | 77% | 0.3125 | 7.94 | general use |
| 7.9 mm | 79% | 0.3110 | 7.90 | general use |
| 7.8 mm | 84% | 0.3071 | 7.80 | more torque, more broken taps |

## Clearance hole

- Close fit: W (0.3860 in / 9.80 mm)
- Free fit: X (0.3970 in / 10.08 mm)

## Counterbore (socket head cap screw)

- AMES: diameter 5/8" (0.6250 in), depth 0.3750 in

Countersink (flat head socket screw): 82 degrees to 0.7810 in across for a flush head.

## Thread repair (Helicoil / STI drill)

- AMES: 25/64" (0.3906 in / 9.92 mm)
- MISUMI: 25/64" (0.3906 in / 9.92 mm)

## FAQ

Q: What size drill do I use for a 3/8-16 tap?
A: Use a 5/16" (0.3125 in / 7.94 mm) drill. It leaves about 77% thread, the standard for general-purpose tapping.

Q: What metric drill can I use for a 3/8-16 tap?
A: 8.0 mm (0.3150 in / 8.00 mm) is the closest, giving about 74% thread. Anything from about 65% to 80% thread taps well in most materials.

Q: What size clearance hole for a 3/8-16 screw?
A: W (0.3860") for a close fit, X (0.3970") for a free fit.

Q: What is the metric equivalent of 3/8-16?
A: M10 is the closest size (10.00 mm vs 9.52 mm major diameter). The pitches differ, so the screws and tapped holes are not interchangeable.

## Sources

- Engineers Edge: machinist screw thread tap drill chart (ANSI): https://engineersedge.com/tap_drill_chart.htm
- Penn MEDesign: tap drill chart: https://medesign.seas.upenn.edu/index.php/Guides.TapDrillChart
- Fractory: imperial clearance hole sizes: https://fractory.com/?p=21622
- AMES: counterbore hole dimensions, inch (Machinery's Handbook): https://amesweb.info/Screws/Counterbore-Hole-Dimensions-Chart.aspx
- SC Fastening: flat head socket cap screw spec, ASME B18.3 (PDF): https://scfastening.com/wp-content/uploads/2017/06/flatsocketspec.pdf
- Eugene Fastener: flat head socket screw countersink guide: https://www.eugenefast.com/Home/Page/resources/flat-head-socket-screw-countersink-guide
- AMES: Helicoil tap sizes (STI): https://amesweb.info/screws/helicoil-tap-sizes.aspx
- MISUMI: Helicoil tap and drill size chart: https://blog.misumiusa.com/helicoil-tap-and-drill-size-chart/