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The bores of the cylinder block are usually machined prior to assembly with the cylinder head. In this case, bore distortion occurs when the cylinder block is assembled with the cylinder head due to the load applied by the head bolts and the surface pressure of the head gasket. This bore distortion influences sealing and operating characteristics of the pistons and piston rings, requiring an increase in bore thickness and addition of ribs to obtain higher cylinder block rigidity, which lead to an increase in weight. In order to improve engine performance, it is necessary to control bore distortion more effectively. With the aim of reducing bore distortion when assembled with the cylinder head, the bores are machined with a dummy cylinder head installed on the block to provide an equivalent head bolt load and gasket surface pressure. By using this bore circulatory machining technology, bore distortion after cylinder head assembly can be reliably suppressed.