Specifically designed to attach wood to metal (Max 5mm). Coarse self-tapping thread with wings designed to ream out the timber, so the screw can achieve Max R.P.M. without burning out. When the wings hit the metal they will break off to permit the thread to self-tap into the metal. Bi-met screws combine the ultimate corrosion resistance of an A2 stainless steel fastener with the exceptional drilling performance of a hardened carbon steel drill tip.