A gas turbine engine rotor adapted for rotation about a longitudinal axis thereof, the rotor comprising: a plurality of blade supporting disks (10,15,20) adapted for mounting on the rotor by longitudinally compressive preloaded retention in a longitudinal stack; a first disk of the plurality of blade supporting disks (10, 15, 20) includes a medial web and a first annular rim disposed at a radial outer portion of the medial web, the web having forward and aft surfaces which, with adjacent radially inner surfaces of the first annular rim, define a pair of major surfaces of the first disk, the first disk including in a first major surface thereof, a first recess; a second disk of the plurality of blade supporting disks (10, 15, 20) adjacent the first disk and separated therefrom by a gap, the second disk includes a medial web and a second annular rim disposed at a radial outer portion of the medial web, the web having forward and aft surfaces which, with adjacent radially inner surfaces of the second annular rim, define a pair of major surfaces of the second disk, the second disk including in a first major surface thereof opposing the first major surface of the first disk, a second recess; and a spacer having first and second ends received in the first and second recesses (80,85) respectively of the first and second disks (20,15), the spacer spanning the gap and transmitting the compressive preloading from one of the first and second disks (20,15) to the other of the first and second disks (20,15), wherein the first annular rim includes a first shoulder at an edge thereof and a first flange extending longitudinally from the first shoulder of the first disk toward the first major surface of the second disk, and wherein the second annular rim includes a second shoulder and a second flange extending longitudinally from the second shoulder toward the first major surface of the first disk, characterised in that: the second flange is provided with an undercut, and the first flange at an edge thereof is received within the undercut. a plurality of blade supporting disks (10,15,20) adapted for mounting on the rotor by longitudinally compressive preloaded retention in a longitudinal stack; a first disk of the plurality of blade supporting disks (10, 15, 20) includes a medial web and a first annular rim disposed at a radial outer portion of the medial web, the web having forward and aft surfaces which, with adjacent radially inner surfaces of the first annular rim, define a pair of major surfaces of the first disk, the first disk including in a first major surface thereof, a first recess; a second disk of the plurality of blade supporting disks (10, 15, 20) adjacent the first disk and separated therefrom by a gap, the second disk includes a medial web and a second annular rim disposed at a radial outer portion of the medial web, the web having forward and aft surfaces which, with adjacent radially inner surfaces of the second annular rim, define a pair of major surfaces of the second disk, the second disk including in a first major surface thereof opposing the first major surface of the first disk, a second recess; and a spacer having first and second ends received in the first and second recesses (80,85) respectively of the first and second disks (20,15), the spacer spanning the gap and transmitting the compressive preloading from one of the first and second disks (20,15) to the other of the first and second disks (20,15), wherein the first annular rim includes a first shoulder at an edge thereof and a first flange extending longitudinally from the first shoulder of the first disk toward the first major surface of the second disk, and wherein the second annular rim includes a second shoulder and a second flange extending longitudinally from the second shoulder toward the first major surface of the first disk, characterised in that: the second flange is provided with an undercut, and the first flange at an edge thereof is received within the undercut.