Specifically designed to attach wood to heavy section steel (Max. 12mm). Metric 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. NOTE: Partially threaded. Thread Dia. mm: 5.5 Length mm: 120 Head Type: Countersunk with Ribs Recess Type: Phillips Point Type: Self-Drilling No.5 Finish: Zinc Material: Case & Core Hardened Carbon Steel Effective Thread Length mm: 51 P/T Max Drilling Thickness mm: 12.0 To Suit Timber mm: 50: 84Size 5.5 x 120 - 100 pcs Timco Metal Timber to Heavy Section Screws - Countersunk - Wing-Tip in Silver