Piston Ovality - Single and Double Ovality

ovality is put on the pistons od because it actually deforms the piston crown (top) when combustion takes place, and when it has ovality it deforms to the round shape of the bore.

Modern automotive engine all have piston skirts that are 'cam ground' or equivalent cast pistons.The objective is to minimize cylinder to wall contact (reducing friction). The thrust side of the piston is ground/cast to a different diameter and is also oval so that when at operating temperature the piston skirt against the clyinder during the power stroke optimally distributes the thrust load.

New eengines almost all use cast pistons with steel slipper inserts to control thermal expansion. THis enables maintaining very tight piston-to-cylinder wall clearances to minimize any noise fron 'piston slap' when cold. The operating clearance may be under 0.001 inches in the new engine designs. The cold clearance at the top of the piston is much larger as it gets much hotter during operation.

About the only things straight on/in a modern piston are the ring grooves and wrist pin features. Everythig else is built with complex geometry to achieve minimum drag, minimum noise when cold, prevent scuffing when hot, and minimize clearance volume between piston and cylinder above the top ring. Modern high precision casting of pistons and ultra-high precision honing of cylinder bores enables mass production of extremely close clearance bearings/shafts and piston/bores with extremely high repeatability. It is all but impossible to 'rebuild' one of these new engines to precision clearances matching those achieved at the factory.