Fill port and pass through fittings allow a custom water cooling builder to install fittings in places that are not normally possible. Fill port fittings are simply pass through fittings labeled for a specific purpose. When a pass through fitting is installed in a location above a reservoir and attached with a tube it allows the fluid to be filled without opening the PC case at all. This is called a fill port. Pass through fittings take several forms. The simplest version is what is used for a typical fillport. Designed to be installed in a hole of a specific diameter, the fitting is held in place with a nut on one side and usually has standard G1/4 thread inside. This allows fittings to be used on either side of a flat surface like a PC case top or side. Another type of pass through fitting is made to be installed in the expansion slot area of the PC, allowing fittings to be installed on either side for external tube routing.