A structure is mounted in a water jacket for a cylinder block of an engine so as to surround a block bore. The structure includes a body forming a pad mounting portion being a panel formed in an arc shape to have inner and outer curved surfaces that are surrounded by the block bore. A mounting hole is extended through the inner and outer curved surfaces of the pad mounting portion. A pad is formed in a shape to correspond with the mounting hole. The pad has a curved surface and is inserted into the mounting hole in a water tight manner.