A side wall closing the casting space of an installation for continuous casting of thin metal strips between unwinding cooled walls, including a support plate, on the rear face of which thrust members exert their action, and an active face arranged against the front face of the support plate. Lateral portions are designed to be worn by contact with the edges of the cooled walls over a maximum depth “e” and a length “1”. The support plate and the active part are urged in contact with each other in a way enabling them to be disengaged non-destructively. The support plate front face has in its central part a recess whereof the depth is greater than “e” and the outer edges of the front face have a general shape which matches that of the outer edges of the active part and have a length less than “1”.