Originally Posted by
WizardOfBoz
Just a point of correction: the nominal correct size for an M6 cutting tap is often given as 5mm. That's for 100% thread depth. As a nod to practicality, most holes drilled for tapping specify about 65-70% of full thread depth. That's where you 5.2mm comes in ...
A 5 mm drill is the standard tap drill for M6 x 1 threads:
4.7 mm drill => 100% threads
5.0 mm drill => 77% threads
5.2 mm drill => 61% threads.