Abstract: A shroud for a gas turbine engine rotor stage is divided into a number of arcuate segments each of which is a trapezium. The segments are urged together by compression springs which attempts to make the assembly lengthen peripherally, by virtue of the chamfered ends 30 of the segments riding up each other. The completely circular shape of the shroud prevents that movement but as a result, a hoop load is generated which tends to move the segments radially outwards, thus achieving the same effect, along with a consequent increase in diameter. A cam ring and pins are used to control and reverse the movement, as desired. The shroud diameter can thus be altered to suit rotor expansion and contraction.