Within the internal combustion engine, the pressure produced by burning the air/fuel mixture in the cylinder must be made to work. The piston does this. It is required to be as gas tight as possible in the cylinder bore yet still be able to slide freely and not to wear either itself or the cylinder bore too rapidly. Initially, pistons were made of cast iron, as in the 1899 Daimler. This is a metal with good bearing qualities when sliding in the cast iron cylinder. Unfortunately it is too heavy for high speeds, particularly as the piston has to change direction at each end of the stroke. By the end of the cast iron era, some very fine thin-section castings were being made but even these were too heavy. Steel was considered and examples of welded-up and forged or pressed steel pistons were subsequently made and used.
For a long time manufacturers were wary of using aluminium because it has an expansion with temperature of about two-and-a-half times that of cast iron, although it is only about a third of the weight. Some firms, like Ferrari and Bentley, used pistons with aluminium tops and cast iron skirts, to try to obtain the benefits of both metals, an expensive form of manufacture, which goes some way to explain the high cost of a new Ferrari for sale. Eventually, during the 1920s, aluminium became more generally used, aided by new alloys and a large number of designs to compensate for the different expansion rates.
As the aluminium of the piston expands more than the iron of the bore, it is essential to provide enough clearance, when the engine is cold, to allow for this expansion. The excessive clearance makes itself heard as piston slap when the piston rattles in the bore with the changing direction of force from the connecting rod.
Some of the schemes suggested to alleviate this trouble were to make the piston oval, to split the skirt so that it could move inwards as the piston grew larger and to fit spring-backed piston rings towards the bottom of the skirt. Mohle K-G of Germany introduced the Autothermic piston which had inserts of another metal cast between the gudgeon pin bosses and the walls so that the differences in expansion moved the skirt to keep the original clearance.
