9 THINGS YOU NEED TO KNOW…
In terms of weight, cost and ease of production, aluminium is the material of choice, bar none. The aluminium used is alloyed with other materials to enhance its suitability for use in such an extreme environment. Copper, nickel and magnesium are used in small amounts (less than 1%) to improve casting and machinability but the key alloying agent is silicon. The silicon content may vary from as little 2% to around 20%. Higher silicon content reduces the rate at which the piston expands. This is important in modern production engines which require the piston to run in its bore with minimal clearance to reduce emissions by better controlling oil consumption. A low silicon content alloy will dissipate heat more efficiently and is less brittle and therefore less likely to break in event of detonation occurring, making low silicon alloys the preferred choice for racing.
Pistons can either be cast or forged. The majority of production pistons are cast by pouring the moulten material into a mould. With forging the addition of huge pressure to the process squeezes the moulten metal producing a finer microstructure, enhancing its integrity. Both types of production require finish machining but casting allows for more intricate shapes to be produced, reducing the amount of machine time required. Consequently forged pistons are significantly more expensive to produce.
Pistons are not the simple cylindrical shape that they appear to be at first glance. Most taper from top to bottom to allow for the greater expansion at the solid piston crown and higher temperatures in this area. Similarly, due to the presence of material across the piston around the gudgeon pin, they may not be round either, requiring an elliptical shape from cold to ensure near perfect roundness and best possible fit in the cylinder at operating temperature.
The top of the piston, profiled to achieve optimum compression combustion chamber shape, often features recesses for valve clearance.
Immediately below the crown, machined to accept the compression and oil control rings and oil drain holes. This area of the piston is critical for good piston-tobore sealing.
Connects the piston to the small end of the conrod. Usually runs as a plain bearing in the piston. Lives its life on the very edge of destruction.
The area below the ring lands, usually cut away to the sides, ie adjacent to the piston pin boss, to reduce weight. The thrust faces to the front and rear prevent the piston rocking in its bores and transfer heat to the cylinder wall.
Or ribs, reinforce the piston and help control expansion and shape.
Piston speed is generally the limiting factor in the quest for more revs and power. All the current crop of top-end sportsbikes have a mean piston speed of around 22 metres per second at peak revs. They will overrev to around 25 metres per second – the accepted safe limit for production engines. More extreme parameters can be set, but reliability and service intervals would not be suitable for production engines.