these pistons require a larger piston to cylinder bore clearance at assembly temperatures. This leads to a condition known as "piston slap" which is when the piston rocks in the cylinder and it causes an audible tapping noise that continues until the engine has warmed to operational temperatures, expanding the piston and reducing piston to cylinder wall clearance. Forged versus cast When a piston is cast, the alloy is heated until liquid, then poured into a mold to create the basic shape. After the alloy cools and solidifies it is removed from the mould and the rough casting is machined to its final shape. For applications which require stronger pistons, a forging process is used. In the forging process, the rough casting is placed in a die set while it is still hot and semi-solid. A hydraulic press is used to place the rough slug under tremendous pressure. This removes any possible porosity, and also pushes the alloy grains together tighter than can be achieved by simple casting alone. The end result is a much stronger material. Aftermarket performance pistons made from the most common 4032 and 2618 alloys are typically forged. Compared to both 4032 and 2618 alloy forged pistons, hypereutectic pistons have less strength. Therefore, for performance applications using boost, nitrous oxide, and/or high RPMs, forged pistons (made from either alloy) are preferred. However, hypereutectic pistons experience less thermal expansion than forged pistons. For this reason, hypereutectic pistons can run a tighter piston to cylinder clearance than forged pistons. This makes hypereutectic pistons a better choice for stock engines, where longevity is more important than ultimate performance. Some vehicles do use forged pistons from the factory. Dodge Vipers used forged pistons from 1992-1999 model years, then switched to hypereutectic. The last generation of Vipers(2013-2017) used forged pistons. All Honda S2000s use forged pistons. See also Y alloy Hiduminium Engine technology Aluminium–silicon alloys aluminium alloys Low thermal expansion materials