A running clearance system for gas turbine engines, comprising: one or more first rotor assemblies (30;164) of a gas turbine engine compressor (14;160) rotatable around an axially extending rotational axis, each first rotor assembly (30;164) having a plurality of blades (32;166) with each blade having a tip (42;174); one or more first stator assemblies (28;162) of said gas turbine engine compressor (14;160) disposed adjacent said one or more first rotor assemblies (30;164); one or more first shroud segments (52;180) having blade seal surfaces (54;182) disposed radially outside of said one or more first rotor assemblies (30;164); one or more second rotor assemblies of a gas turbine engine turbine rotatable around said axially extending rotational axis, each second rotor assembly having a plurality of blades with each blade having a tip; one or more second stator assemblies of said gas turbine engine turbine disposed adjacent said one or more second rotor assemblies; one or more second shroud segments having blade seal surfaces disposed radially outside of said one or more second rotor assemblies; wherein said blade tips and said blade seal surfaces have mating geometries and a clearance distance extending between said blade tips and said blade seal surfaces; and an actuator selectively operable to axially move said first rotor assemblies (30; 164) and said second rotor assemblies to alter said clearance distance; characterised in that said actuator includes a ball screw assembly, wherein said ball screw assembly is mounted such that both said first rotor assemblies (30;164) and said second rotor assemblies move axially in unison when at least a portion of the screw thread device rotates, said portion of the ball screw assembly configured to axially move the outer race of a shaft bearing. one or more first rotor assemblies (30;164) of a gas turbine engine compressor (14;160) rotatable around an axially extending rotational axis, each first rotor assembly (30;164) having a plurality of blades (32;166) with each blade having a tip (42;174); one or more first stator assemblies (28;162) of said gas turbine engine compressor (14;160) disposed adjacent said one or more first rotor assemblies (30;164); one or more first shroud segments (52;180) having blade seal surfaces (54;182) disposed radially outside of said one or more first rotor assemblies (30;164); one or more second rotor assemblies of a gas turbine engine turbine rotatable around said axially extending rotational axis, each second rotor assembly having a plurality of blades with each blade having a tip; one or more second stator assemblies of said gas turbine engine turbine disposed adjacent said one or more second rotor assemblies; one or more second shroud segments having blade seal surfaces disposed radially outside of said one or more second rotor assemblies; wherein said blade tips and said blade seal surfaces have mating geometries and a clearance distance extending between said blade tips and said blade seal surfaces; and an actuator selectively operable to axially move said first rotor assemblies (30; 164) and said second rotor assemblies to alter said clearance distance; characterised in that said actuator includes a ball screw assembly, wherein said ball screw assembly is mounted such that both said first rotor assemblies (30;164) and said second rotor assemblies move axially in unison when at least a portion of the screw thread device rotates, said portion of the ball screw assembly configured to axially move the outer race of a shaft bearing.