Metric Thread - Drill & Tap Chart

The tap drill is the hole you drill before tapping. The rule of thumb is drill = major diameter minus pitch, so an M10 × 1.5 thread takes an 8.5 mm drill and an M8 × 1.25 takes 6.8 mm. That gives roughly 75% of full thread depth, which is close to the full strength of the joint at a fraction of the tapping torque. Going smaller does not make the thread noticeably stronger, it just makes the tap harder to turn and easier to snap, particularly in stainless or in blind holes where swarf has nowhere to go. In those cases it is legitimate to go up a size, for example 8.6 or 8.7 mm for M10.

Clearance holes are for a bolt to pass through, not to be threaded. The three ISO 273 series are close (dowelled or precisely located joints), medium (the general-purpose choice, and the one to use if in doubt) and free (where tolerance stack-up or paint build needs absorbing). Only the close and medium sizes are in the table; the lookup above shows all three. Coarse is the default pitch for a given diameter. Fine pitches are what you will find on sensors, brake and fuel fittings, hub nuts and anywhere a shallow thread or fine adjustment is wanted.