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An oil pan is a component that typically seals the bottom side of four-stroke, internal combustion engines in automotive and other similar applications. While it is known as an oil pan in the U.S., other parts of the world may call it an oil sump. Its main purpose is to form the bottommost part of the crankcase and to contain the engine oil before and after it has been circulated through the engine. When an oil pan is removed, some components revealed usually include the crankshaft, oil pickup, and the bottom end of the dipstick. Some oil pans will also contain one or more magnets that are designed to capture small pieces of metal before they can plug the oil filter or damage the engine.
During normal engine operation, an oil pump will draw oil from the pan and circulate it through the engine, where it is used to lubricate all the various components. After the oil has passed through the engine, it is allowed to return to the oil pan. In a wet sump system like this, the amount of oil that an engine can hold is directly related to the size of the oil pan. An engine can hold no more oil than can fit in the pan without reaching the crankshaft, since a submerged crankshaft will tend to aerate the oil, making it difficult or impossible for the oil pump to circulate it through the engine.
The drain plug used to change the engine oil is typically located somewhere on the oil pan. An easy way to locate an oil drain plug is to find the pan and then look for its lowest point. The pan may be slanted, have a bulge on one end, or be at a slight angle due to the position of the engine. This low point is usually where the drain plug is located so that nearly all of the oil in the pan can be drained through it.
Certain engines, such as those in race or high performance cars, may make use of what is known as a dry sump system. Instead of storing all the oil in the crank case, these engines have a divorced reservoir that it is pumped to and from. Oil pans on engines like these will typically be much smaller than those in wet sump systems, since the oil is returned to the reservoir after being used for lubrication.