This 2 Set includes the 125mm kit and the 230mm kit. These Shrinking Discs work on aluminium and steel and the secret to them is that they create heat through friction when spinning and touching the metal. When the area is cooled by either a squirt of soapy water or compressed air the heated area cools and shrinks. Included discs are in 2 sizes, the larger 230mm one is suitable for use on a 7″ or 9″ grinder or sander and the smaller 125mm one is suitable for 5″ grinders. This kit includes the rubber sanding disc that you require to support the disc and also the tapered nut to attach the disc to your grinder or buff.