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.

* All information without guarantee.

The required core hole depth depends on the recommended thread length. The recommended thread length must be adhered to in order to obtain a secure tight screw connection. The table below shows the thread length from min. to max. A shorter thread means that the screw connection does not meet the required connection force. A longer thread has technically no added value for the screw connection and does not result in a higher connection force.