sounds coincidental that the screw was loose at the same time the haed gasket went, as for the tapping probably a weak lifter or a failing cam lobe, also i think 20 w 50 on a stock motor is not the best choice, more of a racing application. and do you mean 1/2 turn farther past the original adjustment? if so thats probably i little tight and the valves will be at the point where they might not seat properly on the down side of the cam, and it will wear quicker than ever that way.