Black cap. Comes as complete kit with vented dipstick, filter, braided clear PVC covered line. M8 Softail models have the oil tank mounted under the transmission. The engine, with two massive pistons going up and down, breathes in to the oil tank. This system is a way to vent the oil tank to help reduce pressure build-up and reduce power robbing pressures and stops dipstick blow-out. Note: Gaskets are supplied where required.