MTD KH-20-874-15-S  Piston Kit with Rings and Pin Assembly, 94mm

The Piston is the moving component contained within the engine cylinder, the purpose of which is to transfer force from the expanding gas in the combustion chamber to the crankshaft via a piston and/or connecting rod. A damaged piston can be indicative of a damaged cylinder, and may require replacement of the whole engine. However pistons are often made of softer metal, so it's possible there's no damage to the cylinder. Ensure the piston is the only damaged component of the engine cylinder when diagnosing.