The compressor casing, cylinder covers and piston rings are generally of cast iron. Pistons may be of cast iron, steel of aluminium. Aluminium being the preferred material for use on the LP piston due to its larger diameter. Valves are usually made so that parts can be interchanged between the suction Seats are of mild steel with small diameter air passages to prevent the fragments of broken valve plate from entering the cylinder. Valve plates are of vanadium steel heat treated and ground to provided the required hardness and surface finish. Springs should be arranged such that they lift and seat squarely. Uneven spring force or deposits on the seat cause valves to bend resulting in fatigue cracking. For compressors designed for starting air requirements a water jacket relief valve is fitted.