A threaded core hole is a hole into which an internal thread is cut. The core hole must always be slightly smaller than the nominal diameter of the thread to be cut.

As a rule of thumb for the core hole the following applies:

Thread Nominal diameter - Thread pitch = drill size diameter (Core hole diameter or hole size)

Example: M8 tap drill size

8mm - 1.25 mm = 6.8mm (M8 tap drill size)

A distinction is made between standard threads and fine threads for the drill size diameter. The tap drill size table here shows the core hole size for metric standard threads. The thread tap drill size table for fine threads can be found here.

The thread length should be at least twice and at most three times the nominal thread diameter in order to obtain a tight screw connection. The core hole depth must therefore be slightly more than the desired thread length so that the tap can cut the thread completely. A thread length of more than three times the nominal thread diameter should be avoided, as it does not lead to a better screw connection and has real disadvantages. One should know that 3 to 4 threads bear the full load when screwing! All other threads are so to speak without function.

For example: "M3 core hole". An M3 thread has a nominal diameter of 3mm. This results in a thread length of 6mm to 9mm for (2 x d). As core hole depth you can choose 9mm here.