In a piston engine, the main bearings are the bearings on which the crankshaft rotates, usually plain or journal bearings. The bearings hold the crankshaft in place and prevent the forces created by the piston and transmitted to the crankshaft by the connecting rods from dislodging the crankshaft, instead forcing the crank to convert the reciprocating movement into rotation.
While some small single-cylinder engines have only one main bearing, designed to deal with the bending moment exerted by the crank as a result of the force from the connecting rod, most engines have at least two main bearings, one at each end of the crank shaft and may have as many as one more than the number of crank pins. Increasing the number of bearings in an engine will generally increase the size and cost of the engine, but also provides stability to the crankshaft, which would otherwise have to endure greater bending moments from having the crank pin further from a bearing. The combination of bearing number and crankshaft design can also have an effect on engine balance; a more stable crankshaft will better avoid unbalance from flexing, but increasing the crankshaft length or girth to accommodate an additional bearing increases the rotational inertia of the crankshaft, which can have the opposite effect.
Most modern automotive engines have one main bearing at each end of the crankshaft and another in between each adjacent pair of connecting rod journals, but not all engines conform to this generalization. Notable exceptions include the Ford flathead V-8, which had only three main bearings, and Chevrolet's inline-six cylinder engine, which had in various iterations, three, four or seven main bearings.
When describing a crankshaft design, the number of main bearings is generally quoted, as the number of crank pins is determined by the engine configuration. For example, a crankshaft for an inline six engine will be described as three bearing or four bearing depending on its number of main bearings; The crank pins are not counted in this description. Similarly, when speaking of a crankshaft, the journals are the main bearing journals only. The crank pins are not normally called journals although they form the centre shafts of the big end bearings and are therefore journals in the more general sense.
Main bearings are typically held in place by caps which are bolted on. Two bolts per cap is most common, but some engines may have four or six; engines so equipped are referred to as having "four-bolt mains" or "six-bolt mains." The additional bolts result in increased strength, allowing the engine to withstand higher power output.
In general, 6-bolt main cap designs are considered the strongest under all conditions. In most cases, the caps are held in by four bolts from the bottom extending upward into the block (two on each side of the crankshaft) and two cross-bolts coming from the left and right side pan rails into the side of the main caps to provide additional lateral support at high engine speeds. GM LS engines and aluminum-block Ford Modular engines feature 6-bolt mains.