It’s a wide spread misconception that pistons must be round / cylindrical. The opposite is true,
This is a new Kolbenschmidt R27 piston 68.50 mm:
From the Internet:
Profile: If you roll a piston across a flat surface, you’ll notice it doesn’t roll in a straight line. This characteristic is called profile. Aluminum conducts heat significantly, so pistons are designed with a taper. The top of the piston (near the crown) has a smaller diameter than the bottom (near the skirt). The skirt itself is shaped like a barrel. This taper allows the piston to expand as heat is applied, preventing it from binding in the cylinder bore. The degree of taper is carefully calculated to avoid issues like scuffing or seizure due to heat expansion.
Ovality: Pistons aren’t perfectly cylindrical either. They’re deliberately made oval. If you measure the diameter of a full-skirted piston from an older air-cooled engine, you’ll find it’s slightly greater front-to-back than side-to-side (parallel with the wrist pin). This oval shape accommodates the varying temperatures near the dome and skirt of the piston, ensuring proper expansion without binding.
So, while a piston isn’t perfectly round, its sophisticated design optimizes performance and durability in the demanding environment of an internal combustion engine.