Harvey is just a little off... . the term billet usually refers to a sold piece of metal the way it comes from the mill... . solid steel, solid aluminum or other allow... . a head or block would start as a solid piece of steel or aluminum and the machining center would carve out the part... . The normal method would be to either sand cast or lost wax cast the part, thus a much less expensive way... and the part is than machined...
A cast part machined is made at a fraction of the cost as a billet piece of goods... . most airplane parts are from billet material where they might machine away 70% of the base metal... a brake rotor on a truck might start as 50 lb chunk of steel before it becomes a rotor were a cast rotor might start as a 22 lb piece and end up as a 20 lb part.....
Almost all aluminum parts, either cast or billet go through a process called vacuum impregnation... this process assures there are no leaks... mag wheels, cylinder heads, water pumps, thermostat housings, etc. etc... . its at least a page to explain vacuum impregnation but almost all aluminum that holds pressure or has a liquid against it has this process... where only some cast iron parts have this process done...
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