Abstract: This invention relates to high silicon, aluminum base, casting alloys having high strength in the as-cast and room temperature aged condition. The alloys of the preferred composition ranges are similar to the F-132 permanent mold, aluminum piston alloy, containing 9.5% silicon, 3.0% Cu, and 1.0% Mg, except that the Mg has been increased to about 3.0%, and Zn has been added to about 5.5%.The addition of the Mg and Zn results in their room temperature aging characteristics, to Brinell hardnesses of between 95 and 120. These hardnesses are comparable to those of the F132 alloy, 100 to 110 BHN, when heat treated at 400 F for from 7 to 9 hours. In addition, the alloys of the invention exhibit a minimum of growth during the hardening cycle, whereas the presently used high silicon alloys exhibit growth rates of 0.001" per inch, and greater, during their heat treatment cycles.