Pro X pistons are the premier cast piston made from a specialized low-expansion, low-friction aluminum alloy, commonly used by Japanese OEM engine manufacturers to give the piston an incredibly low co-efficient of friction. For you, this translates to a cast piston perfect for 2 strokes and 4 strokes that has the durability of a forged piston but at a much lower cost. Certain models come with a tin-based skirt coating that reduces wear during and after the wear-in process. This black coating also comes in handy during engine servicing since any excess wear to the piston skirt or cylinder wall will be very apparent in the wear patterns of this coating.