It is rare to find a motorcycle engine having a smooth surface. Most of the time they have thin plates on their surface, they are called fins. Since motorcycle engines are air cooled, fins increase the surface area of engine casing and enhances heat transfer to the environment, keeping the engine cool. Air cooled engines involve convective heat transfer, an increase in surface area increases rate of heat transfer. On the other hand, car engine casings have smooth surfaces because they are water cooled and do not involve convective heat transfer.

Fins on Motorcycle Engine | Heat Transfer in Motorcycle Engine