Under a microscope even the smoothest engine components have a surface that looks like a ploughed field. If these surfaces made rubbing contact without a film of oil between them, they would grind together, overheat and destroy themselves.

To prevent this happening, all engines have a built-in lubrication system that pumps, sprays or drips a constant supply of oil on all the moving metal components.

In addition to reducing friction, engine oil has the vital task of helping to cool components such as pistons and valves that are in direct contact with the blow-torch temperatures within the combustion chambers. It also helps to make a gas-tight seal between the piston rings and cylinders, and carries away harmful combustion waste products.

How the oil circulates

The path that the oil takes through the engine is shown in the illustration. From the sump, a reservoir under the crank- case, it is drawn through a wire strainer into the pump. Most pumps have an output of several gallons per minute and can produce pressures of more than 6opsi (4.2kg sq.cm). A relief valve limits the pressure in the lubrication system, usu-ally to between 40 and 6opsi (2.8 and 4.2kg sq.cm).

From the pump, oil passes through a filter and into a main gallery drilled in the side of the crankcase. Drillings connect the gallery to the crankshaft main bearing housings and, once the engine is running, oil under pressure is forced between the rotating crankshaft journals and the bearings.

The crankshaft will be hollow or drilled so that the oil supply to the main bearings is also linked to the big-end bearings at the bases of the connecting rods. In this way all crankshaft bearings are pressure-fed.

The connecting rods take the oil a little further. The rod may be drilled near its base so that a jet of oil sprays the cylinder walls and the underside of the piston as the crankshaft rotates.

Alternatively the connecting rod may be drilled along its length so that pressurised oil from the big-end bearing is taken direct to the gudgeon pin to lubricate it. The surplus then spills out to cool the underside of the piston and cylinder.

The camshaft operates at half engine speed, but it needs effective lubrication because of the high rubbing loads on the cams. It is usual to pressure-feed the camshaft bearings, and to splash or spray oil on the cam lobes.

A push-rod engine with its camshaft mounted low in the crankcase, has oil piped to the hollow rocker shaft, from where it lubricates the rocker bearings and push-rod ends. The surplus then drips down the push-rod openings and coats the camshaft lobes on its way back to the sump.

On engines with an overhead camshaft, two systems are in popular use. In the simplest system the rotating camshaft lobes dip into a trough atoll.

An alternative is to spray the cam lobes with oil. This is usually done by a perforated oil pipe alongside the camshaft. Small hales drilled in the side of the pipe aim a jet of oil onto each rotating lobe. The surplus splashes over the valve assembly before dripping back to the sump.

On cars where an internal chain drives the camshaft, a small tapping from the main gallery sprays oil on the chain links as they move past.