The piston is a plunger that moves back and forth or up and down within an engine cylinder barrel.It transmits the force of the burning and expending gases in the cylinder through the connecting rod to the engine crankshaft.As the piston moves down in the cylinder,during the intake stroke,it draws in the fuel-air mixture.As its move upward,it compresses the charge.Ignition takes place and the expending gases cause the piston to move toward the crankshaft.On the next stroke,it forces the burned gases out of the combustion chamber.
In order to obtain maximum engine life,the piston must be able to withstand high operating temperatures and pressures;hence,it is usually made of aluminum alloy which may be either forged or cast.Aluminum alloy AMS 4140 is often used for gorged pistons.Cast pistons may be made of Alcoa 132 alloy.Aluminum alloy is used because it is light in weight,has a high heat conductivity and has excellent bearing characteristic.
The top of piston is the head.The sides form the skirt.The underside of the piston often contains ribs or other means of presenting maximum surface area for contact with the lubricating oil splashed on it.This oil carries away part of the heat conducted through the piston head.
Some pistons are constructed with slightly oval cross section.The diameter perpendicular to the piston pin is greater to allow for more wear of the piston due to additional side trust against the cylinder walls.Such a piston is called cam ground and must be installed as indicted by a mark on the top of piston head.
Grooves are machined around the outer surface of the piston to provide support to the piston rings.The metal between the grooves is called a groove land or simply a land.The grooves must be accurately dimensioned and concentric with the piston.
The piston and ring assembly must form as nearly as possible a perfect seal with cylinder wall with very little friction.The engine lubrication oil aids in forming the piston seal and in reducing friction.All the piston assemblies in any one engine must be balanced.This means that each piston weight within 1/4 oz (7.09 g) of each of the others.This balance is most important in order to ovoid vibration while the engine is operating.In any case,the weight limitations specified by the manufacturer must be observed.