Abstract: The invention disclosed herein relates to a tightening system including a wrench for applying torque to a fastener system. Associated with the wrench is a control system including measuring means for developing a first signal representative of the torque applied to the fastener. A second signal representative of a constant incremental time is introduced along with the torque signal into gradient calculating means which develops a signal representative of the slope of a torque-time curve which can be plotted for the particular fastener system being tightened. When the gradient signal falls to a predetermined percentage of the maximum previously stored gradient signal, at the yield point of the fastener system or some similarly significant point on the torque-time curve, a control signal is generated stopping the tightening of the fastener.