Fittings are made of many different materials including stainless steel, brass, plastic, Monel, and more. Not always, but often fittings match the material of the conductor used in a system. This multitude of materials creates fittings with a wide array of performance capabilities. Most hydraulic fittings have designated pressure and temperature ratings as well as size and dimension standards set by the SAE.http://www.yinuohydraulic.com/hydraulic-fitting/