Thus most lubrication problems show up on the piston skirt first as scoring and other visible indications. The skirt is the part of the piston most in need of lubrication. There are also stresses involved with rpm that the piston and cylinder are designed together to deal with. It slides against the cylinder wall, bearing the force of combustion on the power stroke, and the loads of compression on the compression stroke. The skirt is the bearing portion of the piston. The pin bosses are typically the thickest part of the piston, and with the crown, the next thickest part, largely determine how the piston reacts to heat, particular the shape the piston will take when hot, as the piston expands the most in these thick areas. This spreads the shock loads, easing stresses on the connecting rod and eliminating a noise called "piston slap." Surrounding the hole inside the piston are pin bosses, thick masses of metal that support the pin when it is inserted in the hole. In road-going engines and those designed for high rpm the hole is offset from the piston's center slightly so that when the piston and rod reach TDC, they do so at slightly different times. This hole accepts a pin that connects the piston to the connecting rod. They support the shock loads the rings receive during combustion. They are similar to the lands in a gun barrel. The solid pieces between the grooves are called ring lands.
This helps the groove stay clean of carbon and ensures uniform ring and cylinder sealing. In the four-stroke engine, natural harmonics cause the ring to rotate as the piston goes up and down in the cylinder. The closely manufactured groove accepts the third part, the precisely made piston ring. Moving down the piston, the next thing is the ring groove. It must therefore be quite thick so as to not collapse or melt. Consequently, the piston's crown is the hottest part of the engine. The piston's top or crown takes the brunt of combustion's forces and heat. The piston is made of essentially seven parts. In fact, nearly 80 percent of the cylinder's excess heat is drawn away by just the piston's rings. Finally, the piston is a heat conductor, transferring some of the cylinder's heat to the outside. The piston is also a seal, as it seals combustion's forces and compression's trapped air. It is a bearing, receiving loads from combustion and transferring them straight and true through the connecting rod to the crankshaft. Following is just a brief outline of the piston's function and construction. It may be the hardest-working part in the internal combustion engine. The piston is under-appreciated, for sure. A person can't be as dogmatic as they would like, and anyone who is insistent just doesn't know the facts. The answer used to be simple, because one type of piston was in fact better than the other. During the 1970s it was a frequently debated topic, rather like engine oil or brake fluid. Which is better, the cast piston or the forged piston? This argument doesn't come up as often as it used to.
The piston is actually a very complex part containing a lot of technology. Knowing these parts' names sets us up for the discussion to follow.
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