The pattern has to be exactly larger, to allow for the shrinkage of the metal as it cools down.
I spent a short while working with patternmakers and we had expansion rulers for that job. Just like a "normal" ruler with the regular graduations, but these were specific for metal type. Use the "Aluminium" expansion rule for that metal and just work to the sizes stated on the plan.
From watching the guys doing the casting work, the sand was only wetted enough to allow the sand to stick together and nothing more! A real art in what they were doing.
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