Chrome. Positive Crankcase Ventilation; a universal chrome plated steel crankvent system for use on all H-D models. Fits regular 3/8" oil lines. The engine can breath out; but not breath in; thus maintaining vacuum in the crankcase at all RPM for improved performance. Note: Primarily designed for bottom vent and custom head vent applications. Can not be used in regular head vent applications due to space required. Tech tip: How active crank vents work. On the down stroke of the pistons the air in the crankcase is compressed. H-D engines; either bottom or head vent models; are not able to vent the required volume. With large displacement engines this problem increases even more. All crank vents let the pressure escape; but also partially allow it to return when the pistons go up. In an active crank vent (crankcase breather) a valve closes; preventing gasses to re-enter the crankcase; creating a stronger partial vacuum inside the crankcase. This can increase horsepower; depending on the engine. Less problems will occur with blown or weeping gaskets and piston compression rings will seal better.