POLARIS 800 CFI standard CAST TOP END PISTON KIT 2008-UP

A hypereutectic piston is an internal combustion engine piston cast using a hypereutectic aluminum alloy with silicon content greater than the eutectic point of 12 weight percent silicon.[1] Most aluminum-silicon casting alloys are hypoeutectic, meaning the silicon content is lower than the eutectic point, and contain relatively fine elemental silicon crystals formed through the eutectic reaction during solidification. In addition to fine silicon crystals, hypereutectic alloys also contain large primary silicon crystals that form before the eutectic reaction. As a result it contains a much higher phase fraction of silicon. Consequently, hypereutectic aluminum has a lower coefficient of thermal expansion, which allows engine designers to specify much tighter tolerances. The silicon content of these alloys is typically 16-19 weight percent, and above this content the mechanical properties and castability degrade substantially. Special molds, casting, and cooling techniques are required to obtain uniformly dispersed primary silicon particles throughout the piston material.