Metal Construction Timber to Light Section Screws - Countersunk - Wing-Tip - Self-Drilling - Bi-Metal - Exterior - Silver OrganicSpecifically designed to attach wood to light section steel (Max. 4mm). 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.