Sorry, I really didn't pay enough attention. The tank is aluminum, it is a "screw machine" part which is turned down from an extruded bar of aluminum. Sounds wasteful, but the aluminum scrap is sold back for recycling at nearly the cost of the bar material. You essentially pay for the material in the final part, power and machine maintainance. I don't recall the exact specs on the aluminum, but it is pretty good stuff, well above beer can level. The crankcases are done the same way, except from a shaped bar (extruded aluminum) of higher strength material to withstand wear.
Sad to say, C*x has disposed of the Screw Machines, so you will never see an American made C*x engine again. I gather that CS tried to duplicate the engine, but couldn't make the quality. The designs are obsolete anyway.