what I'm asking about is the materials used to build the casting
moulds for little model engine crankcases. ones cast in alooominumm.

what sort of head do you need in the aluminium to get a reliable mould
filling?

what aluminium alloys do you use for casting?
which alloys do you avoid like the plague?

how much venting do you guys find necessary?

how long do your moulds last?

Sand casting is probably your first choice, both for cost and
something the home shop can accomplish. Oil bonded sand,
often called petrobond, is the easiest to work with. Water
bonded sand, sometimes called green sand, needs a little
more work to keep it in condition and needs more venting.
It usually isn't necessary to vent petrobond molds for aluminum.

Sand casting is a one shot mold that you simply remold for your
next casting. The pattern lasts a long time.

356 alloy aluminum is a good starting point or you can remelt
aluminum bell housings, etc., to get a feel for how the process
works. Stay away from extrusions and beverage containers
unless you are prepared to experiment with additives.

A couple inches of sprue should give you enough "head",
however, to get complete castings gates and runners will
likely be necessary.

Take a look at this video on YouTube:
http://www.youtube.com/watch?v=NKXj5Ng3X58 and
also by looking at "more from this user" you will find a short
video about investment casting. If you follow the link to my
projects page, there are a couple examples of lost foam and
also a project using gates and runners.

Rod