Flat Head Socket Cap Screws length is measured in their entirety, which is from the top of the head to the tip. A flat head is cone shaped for use in countersunk holes and has a flat top that fits flush with the mated surface. Stainless steel socket cap screw has a plain finish and a flat head with an internal hex drive. Stainless steel resists corrosion and may be magnetic. The flat head is cone shaped for use in countersunk holes and has a flat top that fits flush with the mated surface. The internal hex drive, also known as a hex-socket drive, has a hexagonal slot that requires an Allen key driver, also known as a hex key or Allen wrench.