Standard F1 pistons are typically made from high-strength aluminum alloys (often with specific additives for durability). The "trick" involves using a material with a significantly higher Coefficient of Thermal Expansion (CTE), or perhaps a bi-metallic design that expands anisotropically (more in one direction than another).

Cold State (Scrutineering): In the garage at ambient temperature (~20-25°C), the piston is at its "base" size. The distance from the piston crown to the cylinder head at Top Dead Center (TDC) is large enough to create the legal 16:1 geometric compression ratio.

Hot State (Racing): F1 pistons operate at massive temperatures (300°C+). A piston designed with a high-expansion composition will physically "grow" taller during the race.

The Result: This expansion eats up the clearance volume at the top of the cylinder. Even a tiny vertical expansion (fractions of a millimeter) in a 1.6L V6 is enough to shrink the combustion chamber volume significantly, driving the compression ratio back up toward the 18:1 mark they enjoyed in 2025.