The invention relates to an internal combustion engine. The internal combustion engine is provided with two air cylinder bases (4,5) arranged in a V shape. The air cylinder bases (4,5) are connected with a crankcase (1) through bolt supports (6,7). In order to realize steady oil-tight connection among a bearing hole (19), the crankcase (1), and the air cylinder bases (4,5), the crankcase (1) and the lower part (19) of the bearing hole are provided with interlocking components (23) with synergistic effect in a parting surface (8), and the bolt supports (6,7) respectively pass through ducts (27,28). The ducts (27,28) extend parallel with an air cylinder axis (9) in vertical walls (10,11), and are screwed in threaded holes (15) on the lower sides of the air cylinder bases (4,5). In the parting surface (8), the ducts (27,28) are sealed by a sleeve (26). The invention also relates to a method for installing the sleeve (26).