In the early days of the war the desirabilitv of reducing the weight of aeroplane engines to a minimum, consistent with reliability and efficiency, very soon led designers to consider the advisability of using alloys considerably lighter than cast iron or steel for all possible parts of aero engines. Aluminium alloy crank cases had been used for some years in connection with motor car engines, and it was natural, therefore, that such alloys should come into use for the crank cases of aeroplane engines. Alloys of aluminium suitable for this purpose can be made, having a specific gravity not more than three, and having an ultimate breaking strength very nearly equal to that of cast iron. It was only necessary, therefore, to overcome foundry difficulties in order that such alloys should be generally used.