An impact power tool includes a motor, an output shaft to which a tool bit is attachable for performing work on a workpiece, and an impact mechanism disposed between the motor and the output shaft for converting a continuous torque output from the motor to discrete pulses of torque upon the output shaft, the impact mechanism including a cylinder assembly, a cavity defined within the cylinder assembly containing a hydraulic fluid, and a collapsible member having a first end and a second end opposite the first end. The collapsible member is maintained in a shape coinciding with that of the cavity by fitment within the cavity, with the first and second ends being disconnected from each other. The collapsible member is collapsible to compensate for thermal expansion of the hydraulic fluid during operation of the impact mechanism.