An M8 bolt has a diameter of approximately 8mm, but the defined diameter of metric bolts is actually slightly larger than the actual diameter of the bolt shaft, so an M8 bolt would have a shaft diameter thats slightly under 8mm. The length of an M8 bolt can vary, but one example metric bolt specification is M8-1.0 x 20, which means the bolt has a diameter of 8mm, a pitch of 1.0mm, and a length of 20mm. The maximum head diameter of an M8 hex bolt is 14.38mm, and the maximum head height is 5.45mm.