What shape is most common for the top of the piston in 4-cycle small engines?

The most common shape for the top of the piston in 4-cycle small engines is flat. This design aids in achieving a proper combustion chamber configuration, allowing for better fuel-air mixture flow and efficient combustion. A flat top helps to create a smooth surface that can effectively transfer heat and achieve a consistent seal against the cylinder head.

The flat design also facilitates a more straightforward manufacturing process and is cost-effective compared to more complex shapes. Additionally, flat-top pistons are often used in applications that require high compression ratios, which can improve engine performance. In contrast, other shapes such as conical, spherical, or curved typically serve specific needs in certain engine designs or performance applications but are not as widely utilized in standard 4-cycle engines.