The crankcase ventilating system is used to remove water vapor, fuel vapor and combustion gases (from blow-by past the piston rings) from the crankcase thereby preventing undesirable combinations of these compounds with the engine oil.

Air is driven into the opening of the oil filler pipe cap by the fan draft and, after being cleaned by screening through the wire gauze in the cap, passes down the filler pipe, circulates through the crankcase and leaves by the outlet pipe connecting to the valve compartment rear cover plate. When the car is in motion, movement of the air stream past the end of the outlet pipe creates a partial vacuum, thus aiding in bringing air into the crankcase through the filler pipe cap. The oil filler cap (and, therefore, the pipe itself) must be correctly positioned with respect to the engine.

When a heavy duty oil bath air cleaner and heavy duty filler cap are installed on a car, a special crankcase ventilating system outlet pipe having a metal gauze filtering element is also installed. This prevents the excessively dust-laden air encountered in heavy dust areas from entering the crankcase through the outlet pipe when the engine is not running.