In a piston engine, the main bearings are the bearings on which the crankshaft rotates, usually plain or journal bearings.
All engines have a minimum of two main bearings, one at each end of the crankshaft, and they may have as many as one more than the number of crank pins. The number of main bearings is a compromise between the extra size, cost and stability of a larger number of bearings and the compactness and light weight of a smaller number. Both have advantages in terms of performance, as a shorter and more stable crankshaft will produce better engine balance.
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