To enable a shaft to rotate easily, every bearing has a small clearance—a crankshaft bearing 2 inches (50.8mm) in diameter, for instance, would have a clearance of 0.003in (0.076mm).

When the engine is at rest the heavy crankshaft sits in the bottom of its bearings, leaving most of the clearance at the top. Once the engine is started, for the first few seconds, a thin film of residual oil prevents metal-to-metal contact until oil under pressure arrives from the pump.

Once the bearing is full of oil, the rotating shaft drags oil molecules round with it. As they approach the narrowest clearance near the bottom of the bearing, they pile up and form a wedge which forces the shaft and bearing apart.

 The action of the oil wedge —shown in exaggerated form in the illustrations—means that the crankshaft literally floats in oil once the engine is running In theory, this floating action means that the crankshaft bearings should never wear out, but they do. How does this happen?

One reason, for example, is a low oil level in the sump. When a car with a sump only partly full of oil is cornered, the oil moves away from the pump pick-up and for a few seconds, air instead of oil is pumped to the bearings.

 If this happens often enough, the metal-to-metal contact increases the bearing clearance and eventually most of the oil spills out of the side of the bearing instead of forming the oil wedge that supports the crankshaft. Once this stage is reached the engine makes a rumbling or deep knocking sound on acceleration as the bearing surfaces hammer together, and it is usually necessary to regrind the crank- shaft journals and fit new undersized bearings to get things back to normal.