If you’re unsure about the appropriate countersunk hole size for your ISO flat head fasteners, this concise guide provides you with the precise dimensions you need for your screws and bolts.
A countersunk hole is specifically designed for flat head fasteners, which have a flat top face that tapers down from the top. These holes are used when you want the top of a screw or bolt to be flush with the surface of the material it’s inserted into. For instance, a flat head screw with a countersunk hole sits flush with or just below the surface, ensuring a neat finish. In contrast, a screw without a countersunk hole will protrude above the surface.
To determine the dimensions you need, please refer to the image and chart below. All dimensions are provided in millimeters, except for the countersink angle. °.
For instance, if you have an ISO M10 flat head screw with a normal fit, you’ll require a pilot hole diameter of 11 mm, a countersunk diameter of 22.73 mm, and a countersunk angle of 90
Counterbore Hole Size Chart for Cheese Head Fasteners (ISO)
| Fastener Size (Thread) | Pilot Hole Diameter (Close Fit H12) | Pilot Hole Diameter (Normal Fit H13) | Pilot Hole Diameter (Loose Fit H14) | Countersink Diameter | Countersink Angle (°) |
| M3 | 3.2 | 3.4 | 3.6 | 6.94 | 90 |
| M3.5 | 3.7 | 3.9 | 4.2 | 8.96 | 90 |
| M4 | 4.3 | 4.5 | 4.8 | 9.18 | 90 |
| M5 | 5.3 | 5.5 | 5.8 | 11.47 | 90 |
| M6 | 6.4 | 6.6 | 7 | 13.71 | 90 |
| M8 | 8.4 | 9 | 10 | 18.25 | 90 |
| M10 | 10.5 | 11 | 12 | 22.73 | 90 |
| M12 | 13 | 13.5 | 14.5 | 27.21 | 90 |
| M16 | 17 | 17.5 | 18.5 | 33.99 | 90 |
| M20 | 21 | 22 | 24 | 40.71 | 90 |