How do piston rings work? What is blowby? Why are there three piston rings? What do each of the piston rings do? Typically there are three piston rings, and they serve three major purposes. They keep combustion gases from entering the crankcase, oil from the crankcase from entering the combustion chamber, and they aid in heat transfer between the piston and the cylinder walls. The video details these processes, and also sheds light on how blowby occurs, and what happens when piston rings are worn out.