# M8 tap drill size

URL: https://www.clove3d.com/tools/tap-drill/m8/

Tap drill for M8 × 1.25: 6.8 mm (0.2677 in / 6.80 mm), about 74% thread.

## Thread specs

- Series: ISO metric coarse
- Major diameter: 8.00 mm
- Pitch: 1.25 mm
- Basic minor diameter: 6.65 mm

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

| Drill | % thread | in | mm | Good for |
|---|---|---|---|---|
| 9/32" | 53% | 0.2813 | 7.14 | deep holes only |
| K | 53% | 0.2810 | 7.14 | deep holes only |
| 7.1 mm | 55% | 0.2795 | 7.10 | deep holes only |
| J | 59% | 0.2770 | 7.04 | deep holes only |
| 7.0 mm | 62% | 0.2756 | 7.00 | hard or tough metals |
| I | 67% | 0.2720 | 6.91 | hard or tough metals |
| 6.9 mm | 68% | 0.2717 | 6.90 | hard or tough metals |
| 6.8 mm (standard) | 74% | 0.2677 | 6.80 | general use |
| H | 77% | 0.2660 | 6.76 | general use |
| 17/64" | 77% | 0.2656 | 6.75 | general use |
| 6.7 mm | 80% | 0.2638 | 6.70 | more torque, more broken taps |
| G | 84% | 0.2610 | 6.63 | more torque, more broken taps |

## Clearance hole

- Close (fine): 8.4 mm (0.3307 in / 8.40 mm)
- Normal (medium): 9.0 mm (0.3543 in / 9.00 mm)
- Loose (coarse): 10.0 mm (0.3937 in / 10.00 mm)

## Counterbore (socket head cap screw)

- NBK: diameter 14.00 mm, depth 8.60 mm
- AMES: diameter 14.25 mm, depth 8.00 mm

Countersink (flat head socket screw): 90 degrees to 17.92 mm across for a flush head.

## Heat-set insert hole (3D prints)

- CNC Kitchen: 9.70 mm
- Generic (Albany County): 10.20 mm

## Thread repair (Helicoil / STI drill)

- MISUMI: 8.3 mm (0.3268 in / 8.30 mm)

## FAQ

Q: What size drill do I use for an M8 tap?
A: Use a 6.8 mm (6.80 mm / 0.2677 in) drill. It leaves about 74% thread, the standard for general-purpose tapping.

Q: What inch drill can I use for an M8 tap?
A: 17/64" (6.75 mm / 0.2656 in) is the closest, giving about 77% thread. Anything from about 65% to 80% thread taps well in most materials.

Q: What size clearance hole for an M8 screw?
A: 8.4 mm (8.40 mm) for a close fit, 9.0 mm (9.00 mm) for a normal fit.

Q: What hole size for an M8 heat-set insert?
A: Between 9.7 mm and 10.2 mm depending on the brand: CNC Kitchen 9.7 mm, Generic (Albany County) 10.2 mm. Use your insert's own number.

Q: What is the inch equivalent of M8?
A: 5/16-18 is the closest size (7.94 mm vs 8.00 mm major diameter). The pitches differ, so the screws and tapped holes are not interchangeable.

## Sources

- Engineers Edge: metric ISO tap drill chart: https://www.engineersedge.com/manufacturing/metric-iso-tap-drill-chart.htm
- Penn MEDesign: tap drill chart: https://medesign.seas.upenn.edu/index.php/Guides.TapDrillChart
- Albany County Fasteners: clearance hole chart (PDF): https://www.albanycountyfasteners.com/media/6b/b8/g0/1764944339/clearance-hole-chart.pdf
- AMES: metric clearance hole chart (ASME B18.2.8): https://amesweb.info/Screws/Metric-Clearance-Hole-Chart.aspx
- NBK: dimensions of holes for hexagon socket head cap screws: https://alb02.internal.nbk1560.com/en/resources/other/article/technical-22-dimensions-of-holes-for-hexagon-socket-cap-screws
- AMES: counterbore hole dimensions, metric (Machinery's Handbook): https://amesweb.info/Screws/Metric-Counterbore-Hole-Dimensions-Chart.aspx
- Global Fastener: ISO 10642 hexagon socket countersunk head screws: https://www.globalfastener.com/standards/detailn_182.html
- Eugene Fastener: flat head socket screw countersink guide: https://www.eugenefast.com/Home/Page/resources/flat-head-socket-screw-countersink-guide
- SC Fastening: flat head socket cap screw spec, ASME B18.3 (PDF): https://scfastening.com/wp-content/uploads/2017/06/flatsocketspec.pdf
- CNC Kitchen threaded inserts (3DJake listing, hole diameter D3): https://www.3djake.ch/en-CH/cnc-kitchen/threaded-inserts-m5-short
- Albany County Fasteners: heat-set insert specs (PDF): https://www.albanycountyfasteners.com/media/64/3d/5a/1764619189/heat-set-insert-specs.pdf
- MISUMI: Helicoil tap and drill size chart: https://blog.misumiusa.com/helicoil-tap-and-drill-size-chart/