A surgical instrument, comprising: a surgical end effector; a threaded rotary input shaft; an actuator member in operable engagement with said surgical end effector and selective threaded engagement with said threaded rotary input shaft such that when said actuator member is in an engaged configuration, said actuator member is in threaded engagement with said threaded rotary input shaft such that rotation of said threaded rotary input shaft causes said actuator member to move axially to impart an actuation motion to said surgical end effector and when said actuator member is in a disengaged configuration, rotation of said threaded rotary input shaft will not be imparted to said actuator member; means for selectively moving said actuator between said engaged and disengaged configurations; and a locking system for preventing axial movement of said actuator member when said actuator member is in said disengaged configuration; characterised in that said actuator member further comprises: a first actuator segment supported for axial travel relative to said threaded rotary input shaft and comprising a first thread engagement portion; and a second actuator segment supported for axial travel relative to said threaded rotary input shaft and comprising a second thread engagement portion, said first and second actuator segments (530; 550) being selectively movable relative to each other by said means for selectively moving between said engaged configuration wherein said first and second thread engagement portions (532; 552) cooperate to threadably engage said threaded rotary input shaft and said disengaged configuration wherein said first and second thread engagement portions (532; 552) do not threadably engage said threaded rotary input shaft. a surgical end effector; a threaded rotary input shaft; an actuator member in operable engagement with said surgical end effector and selective threaded engagement with said threaded rotary input shaft such that when said actuator member is in an engaged configuration, said actuator member is in threaded engagement with said threaded rotary input shaft such that rotation of said threaded rotary input shaft causes said actuator member to move axially to impart an actuation motion to said surgical end effector and when said actuator member is in a disengaged configuration, rotation of said threaded rotary input shaft will not be imparted to said actuator member; means for selectively moving said actuator between said engaged and disengaged configurations; and a locking system for preventing axial movement of said actuator member when said actuator member is in said disengaged configuration; characterised in that said actuator member further comprises: a first actuator segment supported for axial travel relative to said threaded rotary input shaft and comprising a first thread engagement portion; and a second actuator segment supported for axial travel relative to said threaded rotary input shaft and comprising a second thread engagement portion, said first and second actuator segments (530; 550) being selectively movable relative to each other by said means for selectively moving between said engaged configuration wherein said first and second thread engagement portions (532; 552) cooperate to threadably engage said threaded rotary input shaft and said disengaged configuration wherein said first and second thread engagement portions (532; 552) do not threadably engage said threaded rotary input shaft. a first actuator segment supported for axial travel relative to said threaded rotary input shaft and comprising a first thread engagement portion; and a second actuator segment supported for axial travel relative to said threaded rotary input shaft and comprising a second thread engagement portion, said first and second actuator segments (530; 550) being selectively movable relative to each other by said means for selectively moving between said engaged configuration wherein said first and second thread engagement portions (532; 552) cooperate to threadably engage said threaded rotary input shaft and said disengaged configuration wherein said first and second thread engagement portions (532; 552) do not threadably engage said threaded rotary input shaft.