This study aimed to understand the mechanics of failure of K3 rotary files and recommend a better application of torque limit for each canal condition leading to an optimum number of file uses. Materials and Methods: Instruments and Devices: 32 K3 rotary files (SybronEndo, Orange, CA) Size 25, taper of 10%, 8%, 6% and 4% were used. TCM Endoll (TCM Endo, Nouvag, Goldach, Switzerland) is a slow speed, electric torque-control microprocessor with a 16:1 handprice was used in this study. The motor settings were as manufacture recommended speed : 350 RPM. Torque control value:2N. cm. Compared to 0.5N. cm. Teeth selection: a total number of 103 extracted mandibular first molars were selected for this study. Only mesial canals with an angle of curvature between 40 and 60 degree with a radius of curvature 2mm and 4mm. Results: effect of each torque control value 2 N.cm on instrumentation: the maximum number of canals instrumented in this group was 24 canals and the minimum was 12 canals. The mean was 16.4. Effect of torque control value: 0.5 Ncm: the maximum number of canals instrumented in this group was 31 canals and the minimum was 13 canals. The mean was 19.6. Conclusion: (1) Low torque values allows an increased number of cancal preparations with a given file. (2) Low torque value provides more cycles to failure of a given file. (3) Low torque value would allow loading interruption (reverse action) that allows stress relaxation. (4) Files taper 4% and 6% are more sensitive torque control value because it adds up on the maximum bending stresses. (5) Low torque value provides less risk level to failure. (6) Torque value can be increased with 10 and 8% taper for speed of the operation and efficiency of dentin cutting with less concern to risk adding up to bending stresses. (7) NCF increases with the decrease of taper.