On metal or similar surfaces, the hole into which the screws will enter must first be drilled with a drill. You can easily do this with self-drilling screws, also known as "smart screws". Thanks to its head, which is like a drill bit, the hole into which the screw will enter does not need to be drilled with a drill beforehand. 70Pcs 3.9x13 60Pcs 3.9x16 50Pcs 3.9x19 30Pcs 3.9x25 70Pcs 4.2x13 60Pcs 4.2x16 50Pcs 4.2x19 30Pcs 4.2x25