In early engines, these covers did not exist. As the rocker arms are critical to having the intake and exhaust valves operate, it was necessary to keep them constantly oiled. With these early engines, the rocker arms would have to be frequently oiled as the oil was constantly being thrown off or contaminated with dirt from the outside environment. The rocker cover was invented to keep the oil in and the dirt out. This part is now found on virtually every existing internal combustion engine today.