A cylinder block assembly may include a cylinder block, a cylinder body disposed in the cylinder block, with a plurality of cylinder bores formed in the cylinder body, a fluid jacket, which is formed between an inner circumferential surface of the cylinder block and an outer circumferential surface of the cylinder body, and through which coolant flows, and a block insert disposed in the water jacket and configured to guide a flow of coolant, wherein the cylinder block may include a second block coolant outlet, which is formed at a second side in a surface of an exhaust side of the cylinder block, and through which the coolant in the water jacket is discharged, and wherein the exhaust side may include a side at which combustion gas is exhausted out of the cylinder body.