Blue cog

A blue cog is a quest item used in the Clock Tower quest to repair the blue wheel. It can be obtained by entering the rat cage. You can do that by going north-east of the building / south of the Camel Cage of the Ardougne zoo, and entering the trapdoor. Follow the pad and go through the wall. Do not try to grab it, since the dungeon rat will bite you. You will need to poison it during your quest.