Key Steel - Metric
Key Steel, or Key bar as it is sometimes called, is an unalloyed length of medium steel bar. It is available in 304mm (1 foot) lengths.
Key Steel comes in both square and rectangular section options. It is most commonly used to lock a component such as a sprocket or gear into a keyway on a shaft.
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.