In a piston engine, the crankcase is the housing for the crankshaft. It is part of the main cavity in the engine, separated from the cylinders by the reciprocating pistons. Crankcase vent filters used on engines separate engine oil gas from fresh air entering the air-box. This way the air filter will stay cleaner and the engine will aspirate fresher air, improving performance of the engine. Crankcase vent filters have vent tubes added to their base. Filters consist of a one-piece design with a rubber or chrome top. Filters with mounting studs can be mounted remotely, but require vent hoses and clamps. Filters mounted direct can be pressed into rubber grommets in valve covers.