Rocker covers, in relation to the internal combustion engine, are covers that are bolted on over rocker arms. They are called valve covers in the United States and Canada. and rocker boxes in the UK.
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.
 Rocker cover gasket
A gasket (rocker cover gasket, or valve cover gasket in the US and Canada) helps seal the joint between the rocker cover and the rest of the engine. Failure of this gasket can cause oil to leak from the engine.