About the size of a coffee mug, the piston moves up and down in a cylinder compressing the air/fuel mixture over and over again to be ignited for power. Then expels the exhaust. Engine valves control what is entering and leaving the engine.
All piston have rings on them that seal the piston to the cylinder wall. The pistons are connected to the crank shaft via the piston rod and pin. The piston is what is responsible for taking the air and fuel and compressing it inside the engine. This is where to term compression ratio comes from. The higher the compression, the stronger the engine.
Piston slap is a common cause of engine noise that you may have heard of. This is usually caused by the skirt of the piston being worn out and when the spark fires the air fuel mixture, it causes the piston to smack against the side of the cylinder wall giving a slap or knocking noise. The pistons do not normally break and should last the life of your vehicle. On some occasions if a timing belt or chain breaks, the valve can hit the top of the pistons and cause it to break. This would require a major engine disassembly to repair.
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