known as the turbine stator 26 referring now to fig2 the high pressure turbine 18 and associated structures are shown in greater detail with the present invention incorporated therein the turbine stator section 26 comprises an inlet vane 28 and intermediate vane 30 the primary function of the vanes 28 and 30 is to properly direct the hot turbine gases against the blades 32 and 34 so that the inertial force of the gases causes turbine rotor section 24 to rotate the efficiency of this transfer of inertial forces is a major factor in the overall efficiency of the engine one means of improving the efficiency of this transfer is to decrease any flow of hot gases between tips of the turbine blades 32 and 34 and the surrounding turbine shroud 22 any gases taking this path transfer very little inertial force to the blades the volume of gases taking this undesirable flowpath is lessened by decreasing clearance between the turbine blade tips and the shrouds 22 and that is the purpose of the present invention the turbine tip clearance is decreased by radially expanding and contracting the turbine shrouds 22 to match the radial expansion and contraction of the tips of the turbine blades 32 and 34 radial position of the shroud 22 is controlled by thermally expanding and contracting relatively massive ring structures 36 37 38 and 39 that extend radially outward from a turbine casing 40 in the embodiment of the invention shown in fig2 compressor discharge air is employed for the purpose of thermally expanding and contracting the rings 36 37 38 and 39 the compressor discharge air is.