A flare nut is the threaded fitting used to connect the copper line sets to the indoor and outdoor units. When tightened properly, it creates a high-pressure, metal-to-metal seal that prevents refrigerant leaks. Many of our kits come with pre-flared copper lines to make this connection easier. It is critical to use a torque wrench to tighten these nuts to the manufacturer's technical specs to avoid over-tightening or leaks. We offer replacement flare nuts in various sizes in our online shop. A secure flare connection is the backbone of a reliable, long-lasting mini-split system. It is one of the most important steps in any ductless mini split installation.