Metric thread clearance

A clearance drill bit will drill a hole for the screw to pass through such as a scope base.

A tap drill bit will drill a hole that you can then tap with the appropriate tap.

What is Tapping?

Drilling and tapping are two distinct actions.

Drilling refers to creating a smooth hole in a material with a drill and motor.

Tapping is the action that creates a thread into the side of the hole.