Unfortunately, almost all Harley-Davidson engines that vent the crankcase through the cylinder heads also expel oil through the same passages. Venting directly into the air cleaner, especially a free-flowing air cleaner, often results in an oily mess on the side of the bike. The banjo fittings & chrome tubes route the vented gasses upward so gravity can keep any expelled oil droplets in the engine where they belong. We recommend venting the crankcase to open air by running the vent line up & out of sight beneath the tank, but an optional catch tank is available for those who would rather vent to a closed container.