On Sat, 10 Jan 2009 11:15:40 -0800 (PST), "
wrote:

356-T6 is usually selected for its HIGH STRENGTH rather than its
ability to pass heat

While I don't disagree with you, my ASME Metals Properties (1954)
shows the 356 alloy to be well above average in thermal conductivity.
What alloy is usually used in aluminum heads?

George