A counterbored hole is usually used for when the top of a screw or bolt needs to be flush with, or sit just beneath, the surface of the material it is inserting.
An example of a pan head and the corresponding counterbore hole is shown in this 3D model:
Use the image and chart below to determine what dimensions to use, all dimensions are in millimetres.
For example, a JIS M8 pan head bolt with a normal fit counterbored hole will require a pilot hole diameter of 9 mm, a counterbore diameter of 18 mm, and a counterbore depth of 6.8 mm.
Counterbore Hole Size Chart for Pan Head Fasteners (JIS)
All dimensions are in millimetres.
Fastener Size | Pilot Hole Diameter (Close Fit) | Pilot Hole Diameter (Normal Fit) | Pilot Hole Diameter (Loose Fit) | Counterbore Diameter | Counterbore Depth |
| M1.6 | 1.7 | 1.8 | 2 | 3.5 | 1.5 |
| M2 | 2.2 | 2.4 | 2.6 | 5 | 2.1 |
| M2.5 | 2.7 | 2.9 | 3.1 | 6 | 2.6 |
| M3 | 3.2 | 3.4 | 3.6 | 7 | 2.9 |
| M3.5 | 3.7 | 3.9 | 4.2 | 8 | 3.1 |
| M4 | 4.3 | 4.5 | 4.8 | 9 | 3.6 |
| M5 | 5.3 | 5.5 | 5.8 | 11 | 4.5 |
| M6 | 6.4 | 6.6 | 7 | 13 | 5.4 |
| M8 | 8.4 | 9 | 10 | 18 | 6.8 |
| M10 | 10.5 | 11 | 12 | 24 | 8.3 |
If you want to learn more about counterbored holes, see our post here.