Redhorse Performance 818 Series tube nuts are used together with 819 Series tube sleeves to create a female AN connection on hardline tubing. This is a great option for a worry-free, long-lasting connection that's popular for brake, fuel, transmission and coolant systems. The tube nuts are machined from aluminum in a variety of AN sizes and anodized color options. They're sold individually, in pairs, or in a set of 6.