Hex Bolts - Metric
A hexagon bolt, or hex bolt to give it its more common name, is a threaded bolt with a plain shank and hexagon shaped head.
Metric is the most common thread type in production today. Usually specified as M then a number x another number. This signifies the thread diameter and the length of the fastener.
For example M4 x 30 means that the major thread diameter (the external distance across the outside of the thread) is 4mm and the length is 30mm. Length is usually measured from the underside of the fastener head.