Clear machined aluminum. Can easily be polished to a mirror finish if desired. External head breathers on H-D models are used to vent crankcases gasses out of the engine. These gasses consist mostly of air; but also dirty oil; water etc. and are routed into the air cleaner to burn excess oil in the cylinders. The non-burned oil is ready to make a mess and will saturate your air cleaner element and leak out of the air cleaner housing. This billet catch can separates the air from the dirty oil and water. Clean air goes into the air cleaner; the rest goes into the catch can housing. The catch can simply be unscrewed from the lid to empty. - CNC machined aluminum - Vented cap - 4 stage internal filtration system - 1 extra replacement o-ring - Dimensions: 1-1/2" diameter x 2-5/8" tall - For 3/8" ID hose