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One of the details that Kurt Urban is very clear about is using a grinder to remove any casting flash or parting lines on the inside surfaces of the block. Aluminum expands at twice the rate of iron and is less rigid, so the metal in the block is experiencing a lot of forces. Any sharp points in the metal have a higher chance of being a "stress riser" or a point where a crack would begin. These stress risers are more critical here than with an iron block. He also doesn't like to have sharp edges that might cut his hand when he's building the engine!