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Heat balance and metal temperatures of 1.5 ℓ and 2.0 ℓ DOHC(Double Over-head Camshaft) engines were measured and compared before and after break-in. The cylinder head break-in and cylinder block break-in were done individually in order to observe the each effect of break-in of them. After the engine break-in, the heat rejection to coolant was decreased and the heat loss through the exhaust gas was increased, but the heat rejection to oil did not show a significant change. The engine metal temperatures were also dropped after break-in. In this paper, those changes after the engine break-in were analyzed with two factors such as the cylinder head and the cylinder block, and it was proved that the changes were resulted from the thermal insulation effect of carbon deposits in the exhaust port and the reduction of friction loss. We suggested a scheme to reduce the size of vehicle cooling system by quantitatively analyzing the effect of the engine break-in.