Pistons are metal blocks (cylindrically shaped) that travel up and down a hollow cylinder in the engine, creating compression. The piston compresses the air and gas inside the combustion chamber. This causes combustion and the piston is forced down. These movements are called "strokes". Cast pistons are usually cheaper than forged pistons, lighter in weight and work well on the street. Zero In Racing has the best quality cast piston sets for 8 cylinder engines from such top high performance brands as Mahle.