Hears the actual weights of cast v cnc billet pistons on Axe Rossi pistons as an example. Cast piston = 3.9 grams, cnc billet = 3.7 grams, The cnc piston has slightly more skirt cut away + two 5 mm holes as well so given this their probably the same weight.The reason to use billet material over cast is said to be stronger + billet they can get higher silicon content. A typical cast piston has around 20-21% max silicon, Billet is up to 30% silicon. The thermal expantion on 30% is less than 20% this is why billet pistons are said to last longer than cast ( the piston expands less so dosnt wear as much in the bore). This is also possibly why theirs more bottom end / acceleration because theirs less friction on the piston in the bore because the piston is expanding less. The thermal expantion on billet 30% silicon material is 15.4, Cast 21% silicon which is the maximum that can be made in cast pistons without compromising the material is 16.3. Obviously any reduction in piston weight will make differences as well. Martin.