A molded cap is disclosed having a sealing ring groove just below the cap handle in order to seal crank case gases in an internal combustion engine. The cap includes two spiral grooves for engaging a respective protrusion formed on the inner annular surface of an oil fill snout which receives the cap. A recess is formed at the end of each spiral groove for receiving a respective protrusion to lock the cap into the oil fill snout. The oil fill snout has a draft angle to provide an outside vertical force component on the cap and sealing ring. The vertical force locks the protrusions into the respective recesses and provides a positive sealing force to the sealing ring when the cap is twisted into the snout and into the closed position. The two spiral grooves increase the resistance to back out and create symmetric force components. At the entrance of each groove, a lead-in angle is used to free the cap from the cover by continuing to turn the cap in a counter-clockwise motion.
