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To build our 509ci engine, the bores on our Big M block measure 4.500 inches in diameter. Besides the capacity to accept such large holes, the block also has the "meat" around them to ensure strength. Dart tells us that even with a monster 4.625-inch bore, the cylinder wall thickness is still a minimum of 0.300-inch. With our smaller, 4.500-inch bores, there's plenty of cushion between the pistons.