The piston, with its connecting rod and bearing, transfers the force of the combustion and expansion of the power stroke to the crankshaft. The piston itself, its rings, and the piston or gudgeon pin are together called the piston assembly. The cutaway shape on this piston allows it to clear the counterweights on this rotating crankshaft. The shape of the piston crown depends on the shape of its combustion chamber, and its compression ratio. In diesel engines, the combustion chamber may be formed totally or in part in the piston crown, depending on the method of injection, so they use pistons with different shapes. The piston crown may be flat, concave, dome, or recessed. The piston must stand up to great heat and pressure. It also must change direction from about 10 times a second to up to hundreds of times a second. In most engines, the weight of the pistons is important for engine balance. This is why pistons should only be replaced in matched sets. Some pistons are forged, while others are cast aluminum alloys. All pistons expand as they heat up. As there is more metal near the gudgeon pin, this area tends to expand the most. To allow for this, many pistons are machined into a slightly oval shape. This is called cam grinding. Then, as the piston heats up and expands, it becomes round. Other methods to control expansion include steel struts or ribs, expansion slots in the skirt, or slots called heat dams that restrict movement of the heat.
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