WebFeb 1, 2021 · In our case, the correct tap drill would be the closest to 3.4 mm from below, and that is the 3.3 mm or 0.13-inch tap drill. Lower percentages such as “75% of the thread depth” are also widely used and considered safe. To help you get the grasp of the fine-coarse difference, the M4 classification comprises of the:-. Coarse 0.7 mm thread.