A dimensionally stable ring element (1, 2) for a heat exchanger casing (3) includes: a cylindrical sleeve (10, 20) having a cylinder axis (102, 202), a first end (104, 204) and a second end (106, 206) remote from the first end, and having an inner wall (108, 208), a dividing wall (11, 21) which projects inwards from the inner wall (108, 208) of the sleeve (10, 20) and extends helically around the cylinder axis (102, 202) along the inner wall (108, 208) from the first end (104, 204) of the sleeve (10, 20) to the second end (106, 206) of the sleeve (10, 20). At its first end (104, 204) the sleeve (10, 20) includes a projection (109, 116, 209, 216) extending parallel to the cylinder axis (102, 202), which projection is arranged in a predetermined circumferential position on the sleeve (10, 20).