The principal function of the piston rings is to form a seal between the combustion chamber and the crankcase of the engine. The goal is to prevent combustion gases from passing into the crankcase and oil from passing into the combustion chamber. During the compression and power strokes, the compression ring seals the combustion gases and prevents blow-by. Although blow-by is not completely eliminated it is kept to an acceptable limit. During the compression and exhaust strokes the cylinder walls are lubricated with oil by throw off from the connecting rod bearings. Excess oil is wiped off by the piston rings. Not only does the oil ring have the responsibility of wiping the excess oil off the cylinder walls, but the job is also shared by all of the rings, leaving a fine layer of oil on the wall to provide lubrication for the following ring.