All aluminum housing fits in crankcase breather line. Prevents oil from being forced out the oil crankcase breather tube. Allows free breathing of engine. Recommended by B. Rutan for installation on VE/LE. Applicable to all homebuilts. Continental – 5/8″ in and out, 1/4″ return. Lycoming – 3/4″ in and out, 1/4″ return.