EK-HDC Fitting G1/4 - Black Nickel 16mm OD EK-HDC Fitting is a solid-tube compression fitting, designed for use with solid tubing such as Acrylic, Copper, Stainless Steel or any other type of compatible hard tube. Use an 9mm allen key to tighten the fitting to ensure a perfect seal every time and to prevent skin damage to the finger tips that competing products may cause Hard Tubing Size: 16mm OD Color: Black Nickel G1/4 Thread