A portable liquid drug delivery device comprising a driving part comprising a driving mechanism and a reservoir part comprising - a reservoir adapted to retain a liquid drug, - an outlet port in fluid communication with the reservoir, - a threaded piston rod with a piston, - an engagement member having an inner thread configured to engage with the thread of the piston rod in an engagement position and to disengage from the thread of the piston rod in a disengagement position, the engagement member being prestressed in the disengagement position, the prestress being adapted to force the engagement member to move from the disengagement position to the engagement position, the engagement member being adapted for only converting in the engagement position a torque of the driving mechanism into a linear movement of the piston rod, the piston being adapted upon the linear movement of the piston rod to deliver the liquid drug to the outlet port, the engagement member comprising a nut, the nut comprising two parts with threads forming the inner thread, the two parts clutching the piston rod in the engagement position such that the threads of the two parts are engaging with the threads of the piston rod, release the piston rod in the disengagement position such that the threads of the two parts are disengaged from the threads of the piston rod, - a handle, the handle being adapted in the disengagement position for blocking the movement of the two parts from the disengagement position to the engagement position, each part comprising an arm, wherein in the disengagement position the handle is located in between the arms and adapted for blocking the movement of the two parts from the disengagement position to the engagement position, in the engagement position the handle is retracted from the arms to the extent that the two parts can take the engagement position. - a reservoir adapted to retain a liquid drug, - an outlet port in fluid communication with the reservoir, - a threaded piston rod with a piston, - an engagement member having an inner thread configured to engage with the thread of the piston rod in an engagement position and to disengage from the thread of the piston rod in a disengagement position, the engagement member being prestressed in the disengagement position, the prestress being adapted to force the engagement member to move from the disengagement position to the engagement position, the engagement member being adapted for only converting in the engagement position a torque of the driving mechanism into a linear movement of the piston rod, the piston being adapted upon the linear movement of the piston rod to deliver the liquid drug to the outlet port, the engagement member comprising a nut, the nut comprising two parts with threads forming the inner thread, the two parts clutching the piston rod in the engagement position such that the threads of the two parts are engaging with the threads of the piston rod, release the piston rod in the disengagement position such that the threads of the two parts are disengaged from the threads of the piston rod, - a handle, the handle being adapted in the disengagement position for blocking the movement of the two parts from the disengagement position to the engagement position, each part comprising an arm, wherein in the disengagement position the handle is located in between the arms and adapted for blocking the movement of the two parts from the disengagement position to the engagement position, in the engagement position the handle is retracted from the arms to the extent that the two parts can take the engagement position.