What is the counterbore size for M6?

M6 socket head cap screw: clearance hole φ6.6mm, counterbore φ11mm, depth 6.5mm.

What is the difference between counterbore and countersink?

A counterbore has a flat bottom (for socket head cap screws), while a countersink is conical at 82° or 90° (for flat head screws).

Socket head cap screw (ISO 4762) counterbore, clearance hole, and countersink dimensions.

Common:

Bolt Size	Clearance Hole		Counterbore (SHCS)		Countersink (FHS)		Head Dia dk	Head Height k
Bolt Size	Normal	Close	Dia φ	Depth	Dia φ	Depth(90°)	Head Dia dk	Head Height k