By Doug Kaufman, editor of Engine Builder magazine, a sister publication to Underhood Service.
Figure 1: Overplated copper alloy bearing gouged by cast iron debris. Inset photo shows the microscopic detail of the gouges. (Courtesy of Federal-Mogul)

Engine bearings have the dual function of reducing friction between a rotating part of the engine (the crankshaft) and the stationary part (the main caps and engine block) and supporting the crank.

Because of the stresses caused by the explosions inside the internal combustion engine, the bearing material must be extremely strong, so a durable metal is required.

Reducing friction is accomplished in part by the fact that dissimilar metals slide against each other with less friction and wear than similar materials will.
Figure 2: Babbitt bearing embedded with machining debris. The inset photo shows microscopic detail of the debris. (Courtesy of Federal-Mogul)

So an alloy bearing material does a much better job of keeping a steel crankshaft moving than a steel or cast iron bearing does.

Although the material itself may give the engine bearing some friction-reducing properties, its performance is enhanced by a lubricant between the moving and stationary surfaces. Another of the bearing’s duties is to establish and maintain a film of oil.

For all these requirements, the bearings usually do...