Abstract: A filler cap assembly for containers of liquids, particularly of those which require periodic replenishing, checking or replacement, such as for rotating machines, internal combustion engines and motors. The filler cap is locked in a vibration-proof manner in its closed position by means of a mechanical spring and bosses arranged exteriorly with respect to the filler neck and is tiltable about bosses on the filler neck into the open position remaining captive with the filler neck. Additional elements can be mounted on top of the filler cap for specific operational purposes.