When I posted the sujestion that you use Al alloys such as pistons, alloy wheels, transmissions etc and that you ingot them, it was based on what I said, a little experience and quite a lot of reading. Most of my information came from
http://groups.yahoo.com/group/hobbicastThere was a break away group that unfortunately no longer exists, there are probably new groups on the web since I started, but I've been reasonably sucessful so far. My casting to date has been mostly things like pulleys and machine tool parts. I've nearly finished a dividing head and have a tool cutter grinder under way as well. I'm not very good at explaining some of the things that go on with the physics of casting. Even though I've been fairly sucessful, I have had gas bubbles in my castings, they weren't a problem for me, but they could be for you.
I'm not interested in "I told you so", it just seemed to me to be a shame to not have as much chance as possible on your side. I've really enjoyed the "Willy mamouth" build up, I would be pleased to have been of help.
Wishing you sucessful casting, Matthew