Metallic piston rings seal the combustion/expansion chamber from the crankcase, and assure the flow of heat from the piston to the cylinder. They also regulate and control the use of engine oil by preventing the oil from passing from the crankcase to the combustion chamber. Most automotive pistons have three rings: the top two, while also controlling oil, are primarily for compression sealing (compression rings); the lower ring is for controlling the supply of oil to the liner, which lubricates the piston skirt and the compression rings (oil control rings).