There is a piston inside the cylinder bore, and the piston ring, as the name implies, is used to seal the piston. What's more, they also have the function of lubrication, regulating a film of lubricate, and the ring can be used to transfer the heat from the piston to the cylinder walls, completing the cooling process in the piston. Specifically, they compress up against the cylinder walls to create a seal for oil and combustion gases. There are two kinds of piston rings: one is the compression rings (top and middle) and oil control rings (bottom). However, there's a process for your piston ring to get wear and tear over the time. You are supposed to do regular maintenance and check if your piston ring works properly. Once you decide to change, we are glad to be your first choice and we offer you all Subaru genuine OEM piston ring set with a discounted price.