Designed for the 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. GM designed this to eliminate any chance of installing the non-vented cap where a vented cap is required. ATTENTION (Our adapter is designed for tank with Left-handed threads). O-rings on the center core seal off the tank vent port for proper testing. Compare to GM factory tool GE-52098-A. See Application tab for details