PURPOSE: To reduce the whole height of a metallic water supply cap in a radiator for compactification, to reduce the number of part item, and to improve stability.

CONSTITUTION: A compression coil spring 7 is arranged between a top part 2 inside face of a hat cross sectional link 4 and the central part, on a pot-shaped bottom inside face of a pot-shaped bottom plate 6. The circumferential edge of a flange part 3 is freely removably brought into contact with an inside flange part 5 of the bottom plate 6 from the inside face side of the inside flange part 5. In this way, the axis directional mounting length of the compression coil spring 7 can be sufficiently secured, so that the height of a water supply cap l can be lowered and compactification can be accomplished when the compression coil spring 7 with the same axis directional length is used.