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You have probably seen them before. The fellow that installed the ductwork for your furnace or air-conditioner probably used them. You may have even used them when putting something together that said on the box Some Assembly Required. They are called self drilling screws and are they ever handy! A special point drills through the steel sheet metal quickly then the hardened steel threads self-tap the hole and pull everything together securely. Zinc plated and stress relieved for strength. 5/16 Hex Head. #3 Point size will drill and tap a combined materail thickness of .110" to .210". Made in Taiwan.