Like the pedals on a bicycle, the pistons are the mechanical components that bear the force that is ultimately turned into vehicle motion. While leg muscles generate the pressure on the bicycle pedals, it is the pressure from combustion in the cylinders that propels the pistons. The pistons in turn drive the connecting rods, which turn the crankshaft, transmission gears and driveline components that end up turning the wheels.

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The piston is the moveable floor of the combustion chamber that rises and falls in the cylinder according to the stages of the 4-cycle internal combustion engine. On the intake stroke the piston descends, creating low pressure in the cylinder while the intake valve is open, and atmospheric pressure forces in the air/fuel mixture. On the compression stroke the intake valve closes and the piston rises, compressing the air/fuel mixture and increasing the temperature of the charge. When the piston is near the top of its travel (known as Top Dead Center, or TDC) the spark plug ignites the mixture and the combustion pressure forces the piston downward. Finally, on the exhaust stroke, the piston rises as the exhaust valve is open, forcing the spent gases out of the cylinder.

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Pistons are cast or forged from aluminum alloy. Hypereutectic pistons are a type of cast piston that is used in high performance engines. They have a higher concentration of silicone in the alloy, which increases their hardness, wear resistance, and durability. The forging process increases strength and resistance to damage, and forged pistons are generally used for extreme conditions like racing. Although pistons are a tight fit in the cylinders, with clearances for some engines at 0.001” or even less, for effective combustion chamber sealing piston rings are used. These fit into grooves near the top of the piston, called the crown, or head, and ride against the cylinder walls. The most common configuration is 2 compression rings with a 3rd ring below them for oil control.

The piston is attached to the small end of the connecting rod with a hardened steel pin, typically called a wrist pin. The pin can be a press fit in the connecting rod bore and free to turn in the piston bore, or it can be “fully floating” and free to turn in each component. Such pins must be retained in the piston with circlips to prevent sideways movement and contact with the cylinder wall. The piston skirt is the area between the bottom ring groove and the bottom of the piston. This is the bearing surface of the piston where it contacts the cylinder wall. Wear in this area, called “scuffing”, can be caused by lack of lubrication or lack of clearance due to bore distortion. To help prevent scuffing, many pistons today come with moly coated skirts. This coating reduces friction and allows very tight piston-to-cylinder wall clearance.

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