A gas turbine engine has a spool including compressor and turbine rotors connected by a first shaft. The first shaft extends concentrically around a second shaft. The first shaft forward end has a portion with an inner diameter of close tolerance with the second shaft. The second shaft has a region of enlarged diameter located axially aft of the compressor rotor but axially forward of the forward end of the first shaft. The region of enlarged diameter has a diameter greater than the inner diameter of the forward end portion of the first shaft to cause the region of enlarged diameter of the second shaft to engage the first shaft in interference in the event that the second shaft is moved axially aft relative to the first shaft more than a pre-selected axial distance.