How big do you want to cast is the main question NeoTech?
And also how big a furnace will you build?
I would say that for starting out, pours of under 2 kilos (5lbs) are plenty, and less is better. Remember, Aluminum takes up about 3 times the volume of brass or steel, so that's about a 6 kilo size pour in those other metals.
Thick walls are good to have unless it's hard to pour because of the weight.
I'd say make yourself one of those from your pipe about 125mm deep, and see how it feels loaded with a kilo and a half of sand on the end of a 70 cm set of tongs.
I don't think you need to turn it down on the lathe. It will waste away some on the outside with progressive melts. Eyeball it and see if it looks and feels right to you.
To break up your scrap bell housings, put them on a wood fire until at a "black heat" (not melted, but hot) and when hot crack them up with a big hammer. Wear face and skin protection because pieces will fly sometimes when you break it up. Take it easy and keep safe. Tap forcibly instead of smash stuff.
Then you can fit pieces in your crucible.
Get David Gingery's book on the charcoal furnace. It's a must read, I think.
Oh, one other very important thing -- make sure you are working with aluminum if using car parts, NOT magnesium.
Before heating a car part in any way, take a small sliver of the material and try to light it with a torch. If it burns bright white, it's magnesium.
I rarely see this mentioned.
Aluminum bell housings will make good castings. There will be a lot of spongy dross, but a lot of good metal, too. The dross protects the melt. Don't skim until ready to pour. I use a stainless steel serving spoon for skimming. Also a 6mm steel rod is very handy to have. When you can dip it into the melt and it comes up relatively clean with a shiny point instead of all glommed up with a ball of aluminum, the melt is ready to pour.
