A split casing (10) of low, coefficient of expansion material, has internal upstream facing slots (12). A semicircular shroud ring (16), of high coefficient of expansion material, carries the vanes (18), and has a tongue (20) sliding within casing slot (12). Casing stop lugs (14) and shroud stop lugs (22) have mutually abuttable surfaces (70,72) to antirotate the shroud within the casing. The distance between abuttable surfaces of the casing is greater than the distance between abuttable surfaces of the shroud ring at room temperature, but the distance are the same at the long time operating temperature.