This item includes replacement O-rings and nylon inserts for the Alphacool Eiszapfen Pro fittings. The wide O-ring is placed inside the Eiszapfen Pro fitting and seals the hard tube. The nylon inserts are only needed for PETG hard tubes. PETG hard tubes are softer compared to acrylic hard tubes and can deform at hot spots. In particular, the connections at the outputs of the CPU and GPU coolers reach higher temperatures, which can contribute to the deformation of PETG hard tubes. These deformations can lead to leaks. The nylon inserts prevent this deformation when used with the Eiszapfen Pro connections.