I also don't think you understand what contaminated metal is... dirt will not harm your metal, and oils will flare off... contaminated means, you're using metals with alloys you're totally unfamiliar with and are prohibitive of producing a quality of cast you desire using the methods you have at hand. Those wheels are chocked full of magnesium and manganese, both will give you problems if not dealt with under strict controls. Not to mention the cast product you do end up getting not only will have gas bubbles but will also be extremely brittle. Pistons and crank case metals will give you similar issues, as well. But... degassing and lower temps with pistons and crank cases will give you a workable product. Those blue cakes will work for these metals. I've suggested you to try that blue cake and yet you adamantly affirm what you're using will do the job... or should, at least, which implies you're not going to go that route. Learning this trade requires experimentation. What won't work every time for you should give you a clue it just may not be what you ought to be using. Those blue cakes are cheap as candy... it won't hurt you to try it.
Inside that furnace you need your crucible encircled by the flame and your melt not exposed to the combustion of gas and air, this is why I suggest to leave the skin on the melt till you're ready to pour, this helps keep the metal protected from the furnace combustion elements. This is true regardless what metals you're melting down. And as for melting wheels to cast into ingot, expecting them to be "cleaner", does not eliminate the difficult alloys, nor does it purify the aluminum. The ingots you make will have the same properties as the metal you've been casting. Shit in... shit out... in other words. Don't know how to explain it any clearer than that...
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