For a common ISO metric coarse thread, a useful starting rule is tap-drill diameter ≈ nominal diameter − pitch. A clearance hole is larger because the bolt must pass through freely rather than form a thread.
Tap drill and clearance hole are not the same thing
The tap drill is the hole made before cutting an internal thread. The clearance hole is the hole in the mating part through which the screw or bolt passes without threading. Mixing the two is a common assembly error.
Common shop-floor reference
| Thread | Coarse pitch (mm) | Typical tap drill (mm) | Normal clearance hole (mm) |
|---|---|---|---|
| M3 | 0.50 | 2.5 | 3.4 |
| M4 | 0.70 | 3.3 | 4.5 |
| M5 | 0.80 | 4.2 | 5.5 |
| M6 | 1.00 | 5.0 | 6.6 |
| M8 | 1.25 | 6.8 | 9.0 |
| M10 | 1.50 | 8.5 | 11.0 |
| M12 | 1.75 | 10.2 | 13.5 |
| M14 | 2.00 | 12.0 | 15.5 |
| M16 | 2.00 | 14.0 | 17.5 |
| M18 | 2.50 | 15.5 | 20.0 |
| M20 | 2.50 | 17.5 | 22.0 |
| M22 | 2.50 | 19.5 | 24.0 |
| M24 | 3.00 | 21.0 | 26.0 |
These are common reference values for general engineering work. Confirm the drawing, thread class, material, tap supplier recommendation and the governing standard before production.
Through holes vs blind holes
For a through hole, chip evacuation is comparatively straightforward. For blind holes, allow extra drilled depth below the required full thread because the drill point and tap chamfer consume depth. A spiral-flute tap is commonly selected for blind holes so chips are lifted back out, while spiral-point taps are commonly used for through holes to push chips forward.
What changes the best drill size?
- Material strength and ductility.
- Cut tap vs form/roll tap.
- Required thread engagement.
- Thread class and plating/coating allowance.
- Hole depth and chip evacuation.
- Tap manufacturer recommendations.
Practical sequence
- Confirm thread designation and pitch from the drawing.
- Confirm whether the hole is through or blind.
- Select the correct drill for the tap type and material.
- Deburr/chamfer the entry lightly.
- Use suitable tapping lubricant and keep the tap square to the hole.
- Check the finished thread with the specified gauge or fastener where appropriate.
This article is general guidance for preliminary engineering and shop-floor understanding. The drawing, applicable standard, machine/tooling OEM instructions, process-specific safety requirements and qualified engineering judgment take priority.