6 Flexible Coupling Ci To Ci Fernco. Fernco Couplings have earned a reputation for consistent, superior performance, giving you full confidence in their quality and reliability. The dimensional flexibility of Fernco couplings ensures leak-proof pipe seals on virtually any pipe material: plastic, cast iron, asbestos cement, clay, concrete, steel, copper and ductile iron. All flexible couplings are clearly marked with part number, size, and pipe material that the coupling will connect to. Fernco makes fast delivery of stock items. For special applications, our custom design service can supply couplings to individual customer requirements.