The pilot hole is machined with a drill that takes into account the expansion allowance from the standard dimensions of the pilot hole diameter.

In the case of a cutting tap, the pilot hole = minor diameter , so check the pilot hole with a minor diameter plug gauge.
The reason why the pilot hole equals the minor diameter is that the thread root of the tap is smaller than the pilot hole diameter. Therefore, the tap does not cut at the root, and the drill diameter used for the pilot hole remains as the minor diameter.

In the case of fluteless taps, after drilling the pilot holes, the internal thread raised by the fluteless taps is tested using a minor diameter plug gauge to change the pilot hole diameter until the dimensions are met.

The pilot hole diameter is different for cutting taps and fluteless taps.