Metric Thread and Hole Size Selection

Tap drill versus clearance hole

A tap drill prepares a hole in which an internal thread will be cut. A clearance hole allows the bolt to pass freely and has no thread. These operations need different diameters, so this tool provides separate tabs for selected coarse metric sizes.

Example

M8 coarse pitch is 1.25 mm. The listed cutting tap drill range is 6.647 to 6.912 mm. The common drill size is approximately 6.8 mm. This range is not a recommendation for one drill size. The medium (normal) series clearance hole for M8 is 9 mm, giving 1 mm nominal diametral clearance.

Limits

Fine pitches and forming taps are outside the scope. Tap drill ranges follow manufacturer application guides, while clearance diameters are nominal ISO 273 series values. Tolerances and coatings change actual clearance. Verify final tooling and hole dimensions against the drawing, material and manufacturing process.

Do not read nominal clearance as a tolerance result

Diametral clearance in the clearance hole tab subtracts nominal bolt diameter from the selected hole diameter. It does not use actual bolt outside diameter or measured hole limits. The result is therefore not a guaranteed minimum clearance in the manufactured joint. Alignment of several holes cannot be established from this difference alone. Position deviations require separate assessment.

Selecting M12 and the medium clearance series from the existing options gives nominal diameter 12 mm and hole diameter 13.5 mm. The manual difference is 13.5 − 12 = 1.5 mm. This is diametral. In an ideal concentric drawing, radial space on one side is half of it, but actual joint axes need not coincide. Do not report radial space and diametral clearance as the same quantity.

The same thread name does not mean the same manufacturing instruction in both tabs. This tool supports preliminary selection. Check the drawing, coating condition, tool type and assembly requirement together. Nominal clearance does not determine joint strength or tightening torque.

Frequently asked questions

How do you calculate the tap drill range?

In this example, for coarse thread M8, the common tap drill is approximately 6.8 mm and the listed hole range before tapping is 6.647 to 6.912 mm. The range comes from a table for cutting taps, and nominal diameter minus pitch is only an approximation. This preliminary selection excludes forming taps and fine pitches, so check the final choice against material and drawing requirements.

Calculate with these values