And a small point...maybe I remembered this wrong... A bolt is a fastener that is held in place by a nut on the other end. A screw is a fastener that is held in place by its own threads. Are we talking about screws in this situation, just different heads?
bolt - An externally threaded, cylindrical fastener with a head at one end and a threaded blunt end at the other. Bolts are designed to fit into non-threaded holes to join parts and are assembled with a nut.
screw - An externally threaded, cylindrical fastener with a head and a threaded flat or pointed end opposite the head. Screws are designed either to fit into a threaded hole or form threads into material.