Socket Screws - Countersunk - Metric
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.
A countersunk head is usually used when no part of the fastener head is required to be protruding above the level of the substrate. Countersunk heads can also be recessed and filled over if a hidden fixing is required. Usually abbreviated to CSK.