Specifically designed to attach wood to light section steel (Max. 3mm). 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. Exterior plating gives a reliable and durable corrosion resistant coating which resists 500 hours salt-spray." Thread Dia. mm: 4.2 Length mm: 38 Head Type: Countersunk with Ribs Recess Type: Phillips Point Type: Self-Drilling No.3 Finish: Exterior (Silver Organic) Material: Case & Core Hardened Carbon Steel Effective Thread Length mm: 19 Max Drilling Thickness mm: 3.0 To Suit Timber mm: Size 4.2 x 38 - 200 pcs Timco Metal Timber Screws - Countersunk - Wing-Tip - Self-Drilling in Silver