Hex head cap screws are manufactured to dimensional standards of ASME B18.2.1 and to various material and mechanical standards of ASTM and SAE. The length of a cap screw is measured from under the head to the extreme end of the screw. The standard thread length is two times the diameter plus 1/4" up to 6" length and two times the diameter plus 1/2" for screws longer than 6". Shorter length screws are fully threaded. Product standards require the manufacturer's head marking to appear on the top of all hex cap screws 1/4" diameter and larger.