A bearing is a component used to reduce friction and to maintain clearance between stationary and rotating components of the engine. Bearings, or bearing surfaces, are located on the crankshaft, connecting rod, and camshaft, and also in the cylinder block. Bearings can be subjected to radial, axial (thrust), or a combination of radial and axial loads. A radial load is a load applied perpendicular to the shaft. An axial load is a load applied parallel to the shaft. Bearings are classified as plain journal or antifriction bearings.

A friction bearing consists of a fixed, non-moving bearing surface, such as machined metal or pressed-in bushing, that provides a low-friction support surface for rotating or sliding surfaces. Friction bearings commonly use lubricating oil to separate the moving component from the mated non-moving bearing surface. Friction bearing surfaces commonly consist of a material that is softer than the supported component.

Friction bearings, because of their soft consistency, have the ability to embed foreign matter to prevent spreading in the engine. Friction bearings also have the ability to conform to slightly irregular mating surfaces. Friction bearings can be integrally machines, one-piece sleeve, or split-sleeve for easy installation and removal.

An antifriction bearing is a bearing that contains moving elements to provide a low friction support surface for rotating or sliding surfaces. Antifriction bearings are commonly made with hardened rolling elements (balls and rollers) and races. A race is the bearing surface in an antifriction bearing that supports rolling elements during rotation. A separator is an antifriction bearing component used to maintain the position and alignment of rolling elements. Antifriction bearings reduce lubrication requirements and decrease starting and operating friction. Reduced friction results in less power required to rotate engine components and increases overall engine output.

The crankshaft is supported by main bearings. A main bearing is a bearing that supports and provides a low-friction bearing surface for the crankshaft. Small engines commonly have two main bearings, one at each end of the crankshaft. Small engines with three or more cylinders may require more than two main bearings to provide additional support to the crankshaft. Main bearings are mounted in the crankcase and can be either friction or antifriction bearings. Antifriction bearings used for main bearings increase the radial and axial load capacity of the engine but also contributes to engine noise and are more costly than journal bearings..

A rod bearing is a bearing that provides a low-friction pivot point between the connecting rod and the crankshaft and the connecting rod and piston. The big end of the connecting rod is connected to the crankpin journal. The small end of the connecting rod is connected to the piston pin. Rod bearings are usually journal bearings (integrally machined, sleeve, or split-sleeve) or antifriction bearings. Most connecting rods for small engines use integrally machined friction bearings.

Antifriction bearings used on connecting rods are precision ground from hardened steel and are commonly used on two-stroke cycle engines. Friction rod bearings are commonly made from nonferrous metals such as bronze, aluminum, and babbitt. A nonferrous metal is a metal that does not contain iron. Bronze is a nonferrous metal alloy that consists of brass and zinc. Aluminum is a nonferrous metal commonly alloyed with zinc or copper. Babbitt is a nonferrous metal alloy consisting of copper, lead, and tin or lead and tin. Babbitt is commonly used on split-sleeve bearings consisting of a steel backing coated with multiple thin layers of babbitt on the load bearing surface. Split-sleeve connecting rod bearing position in the large end of the connecting rod is maintained with an alignment tab. The alignment tab also prevents rotation of the bearing during engine operation.