Most regular engines have 4, 6, or 8 pistons that go up and down in the cylinders. On the upper side of the piston is what is known as the combustion chamber where the fuel and air get mixed before ignited. On the other side is the crankcase which is full of oil. Pistons have rings which serve to prevent the oil from entering the combustion chamber and the fuel and air out of the oil. Pistons are manufactured from lightweight aluminum alloy and are designed to float in the cylinder without contacting the cylinder walls. They float on a thin layer of oil which is just below the rings. If the rings fail however, oil can leak into the combustion chamber and you will notice grey smoke coming from the exhaust. If the rings wear or you lose oil to the engine, the pistons can score the cylinder walls damaging the engine and requiring either a new one or a rebuild.