5/16-18×2 1/2 six lobe pan floor board screw type f fully threaded black phosphate. Thread-cutting type-f threaded screws are used in heavy gauge sheet metal, aluminum, zinc and lead die castings, cast iron, brass and plastic. In stainless steel, material into which the screw is to cut threads should have a lower hardness by 10-20 rockwell hardness. Pan head screws are a common style that can substituted in most applications for round, truss or binding heads.