Cooling structure for internal combustion engine

A spacer (14) fitted inside a water jacket (13) of a cylinder block (11) in an internal combustion engine is set so that a space (±) formed between an inner peripheral surface of the spacer (14) and an inner wall surface (13a) of the water jacket (13) is smaller than a space (²) formed between an outer peripheral surface of the spacer (13a) and an outer wall surface (13b) of the water jacket (13). Accordingly, even if a position of the spacer (14) is shifted in a radial direction, the inner peripheral surface of the spacer (14) comes into abutment on the inner wall surface (13a) of the water jacket (13) at first. Thereby, the abutment of the outer peripheral surface of the spacer (14) on the outer wall surface (13b) of the water jacket (13) is prevented completely. Therefore, hitting sounds of pistons (18) can be blocked by the space (²) between the outer peripheral surface of the spacer (14) and the outer wall surface (13a) of the water jacket (13).