# 5/16-18 tap drill size

URL: https://www.clove3d.com/tools/tap-drill/5-16-18/

Tap drill for 5/16-18 UNC: F (0.2570 in / 6.53 mm), about 77% thread.

## Thread specs

- Series: UNC
- Major diameter: 0.3125 in
- Pitch: 18 TPI
- Basic minor diameter: 0.2524 in

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

| Drill | % thread | in | mm | Good for |
|---|---|---|---|---|
| 7.0 mm | 51% | 0.2756 | 7.00 | deep holes only |
| I | 56% | 0.2720 | 6.91 | deep holes only |
| 6.9 mm | 57% | 0.2717 | 6.90 | deep holes only |
| 6.8 mm | 62% | 0.2677 | 6.80 | hard or tough metals |
| H | 64% | 0.2660 | 6.76 | hard or tough metals |
| 17/64" | 65% | 0.2656 | 6.75 | hard or tough metals |
| 6.7 mm | 68% | 0.2638 | 6.70 | hard or tough metals |
| G | 71% | 0.2610 | 6.63 | general use |
| 6.6 mm | 73% | 0.2598 | 6.60 | general use |
| F (standard) | 77% | 0.2570 | 6.53 | general use |
| 6.5 mm | 78% | 0.2559 | 6.50 | general use |
| 6.4 mm | 84% | 0.2520 | 6.40 | more torque, more broken taps |

## Clearance hole

- Close fit: P (0.3230 in / 8.20 mm)
- Free fit: Q (0.3320 in / 8.43 mm)

## Counterbore (socket head cap screw)

- AMES: diameter 17/32" (0.5313 in), depth 0.3120 in

Countersink (flat head socket screw): 82 degrees to 0.6560 in across for a flush head.

## Thread repair (Helicoil / STI drill)

- AMES: 21/64" (0.3281 in / 8.33 mm)
- MISUMI: 21/64" (0.3281 in / 8.33 mm)

## FAQ

Q: What size drill do I use for a 5/16-18 tap?
A: Use a F (0.2570 in / 6.53 mm) drill. It leaves about 77% thread, the standard for general-purpose tapping.

Q: What metric drill can I use for a 5/16-18 tap?
A: 6.6 mm (0.2598 in / 6.60 mm) is the closest, giving about 73% thread. Anything from about 65% to 80% thread taps well in most materials.

Q: What size clearance hole for a 5/16-18 screw?
A: P (0.3230") for a close fit, Q (0.3320") for a free fit.

Q: What is the metric equivalent of 5/16-18?
A: M8 is the closest size (8.00 mm vs 7.94 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/