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A hypereutectic piston is an internal combustion engine piston cast using a hypereutectic alloy–that is, a metallic alloy which has a composition beyond the eutectic point. Hypereutectic pistons are made of an aluminum alloy which has much more silicon present than is soluble in aluminum at the operating temperature. Hypereutectic aluminum has a lower coefficient of thermal expansion, which allows engine designers to specify much tighter clearances.

The most common material used for automotive pistons is aluminum due to its light weight, low cost, and acceptable strength. Although other elements may be present in smaller amounts, the alloying element of concern in aluminum for pistons is silicon. The point at which silicon is fully and exactly soluble in aluminum at operating temperatures is around 12%. Either more or less silicon than this will result in two separate phases in the solidified crystal structure of the metal. This is very common. When significantly more silicon is added to the aluminum than 12%, the properties of the aluminum change in a way that is useful for the purposes of pistons for combustion engines. However, at a blend of 25% silicon, there is a significant reduction of strength in the metal, so hypereutectic pistons commonly use a level of silicon between 16% and 19%. Special moulds, casting, and cooling techniques are required to obtain uniformly dispersed silicon particles throughout the piston material.

Hypereutectic pistons are stronger than more common cast aluminum pistons and used in many high performance applications. They are not as strong as forged pistons, but are much lower cost due to being cast.

Forged Pistons
Performance enthusiasts and custom engine builders prefer the use of forged pistons because they are more durable and easily customized to any configuration, which is essential in engine tuning. Some engine applications may require a flycut on the piston crown to assist in valve relief, and this can be easily achieved using a forged piston. The manufacturing techniques employed in the making of a forged piston make it stronger, as there is more surface area concentrated onto its shape.

Forged pistons are also easy to manufacture as there is no need for big, heavy and expensive casting equipment as employed in the manufacture of cast pistons. This does not necessarily mean that forged pistons are better than cast pistons. In fact, the technology used in the making of cast pistons is state of the art compared to the method of forging alloys. The difference actually lies in application.