a piston for internal - combustion engines , reinforced at least at either the shoulder of the skirt or the piston boss thereof by a composite reinforcement consisting of a layer of inorganic long filaments and a layer or layers of inorganic staple short fibers or whiskers . the inorganic filaments are one or a combination of any of carbon , graphite , alumina , silicon carbide and glass , while the inorganic staple fiber or whiskers are silicon nitride whiskers , mineral fibers , potassium titanate whiskers , carbon fibers or graphite fibers , or a combination of those whiskers and / or fibers .