Services top-tank left-threaded cap on L5P Diesel Engine with two tank caps. One cap is located on the side of the tank [Vented] with right-hand threads. The other is located on the top of the tank [Non-Vented] with left-hand threads. Designed to eliminate any chance of installing the non-vented cap where a vented cap is required. O-rings on the center core seal off the tank vent port for proper testing.