How Motorcycles

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piston 4 stroke four stroke valve pocket cutout compression dome ring lands oil relief holes skirt wrist pin hole upside down view note the large supports for the wrist pin note wrist pins are also called piston pins valve pocket cutout skirt piston connecting rod oil relief holes head cylinder the skirt on the piston keeps the piston from pivoting on the wrist pin essentially making sure the piston only travels up and down in the cylinder skirt prevents piston rocking skirt areas highlighted in black wrist pin hole view from bottom pistons almost always made out of aluminum alloy are either cast or forged and then machined cast pistons are widely used in stock motorcycles forged pistons are sold by aftermarket companies such as WISECO and Athena and are stronger than cast pistons the valve pocket cut outs exist so that when the piston is rising and falling at the top of it's travel the valves can begin to open or close without contacting the piston as it moves the oil control ring lowest ring scrapes oil from the cylinder wall and redirects it through the oil relief holes in the piston the primary purpose of the oil control ring is to remove the majority of the oil from the wall during the downstroke of the piston so the upper compression rings can seal the combustion chamber a secondary benefit or redirecting oil through the oil relief holes is the oil actually gets sprayed onto parts of the wrist pin remember the piston is moving quite fast the routing of the oil through the relief holes occurs very quickly this way the wrist pin receives additional lubrication it also receives oil that flings off the spinning crankshaft and possibly oil being sprayed from jets at the base of the cylinders the oil coming through the relief holes also sprays on the underside of the piston helping to cool the piston