Radium Engineering 10an ORB PCV VALVE- PCV Valve, 10AN ORB to 10mm SAE MaleRadium Engineering valve for PCV (positive crankcase ventilation) systems. Do not mistake this for a simple one way check valve. The PCV valve's internal plunger dynamically meters the appropriate amount of crankcase vent airflow based on current engine load. Proper PCV valves prevent the "vacuum leak" that a check valve would create at low loads. Also, in the event of a intake manifold backfire, the PCV valve closes preventing combustible vapors from