Tank cap for powered machine

A tank cap for closing the filling port of a tank mounted on a power-driven machine having an internal combustion engine, said tank containing a fuel to be supplied to said engine or lubricating oil, wherein the improvement comprises that said tank cap is made of a transparent plastic material and has a protrusion protruding outwardly from the main surface of said cap, said protrusion defining therein a cavity communicating with the interior of said filling port of said tank such as to allow said fuel or lubricating oil to enter said cavity, and wherein said protrusion is constituted by two parts extending along a common diametrical line of the main surface of said tank cap and separated from each other by end walls defining radially inner ends of said parts of the protrusion, said end walls being made integral with the cap so as to constitute a central stiffening wall.