For decades, automotive connecting rod were fabricated machining separately the cap from the body, with precision to maintain accurate alignment, being that the fixing was by the use of special screws, with grinded body to remain perfectly adjusted in passage holes, aiming this way prevent possible lateral displacements.From the end of the last century, due to new technologies in the production and use of special microalloyed steels such C7056BY, permitted to introduce in the market connecting rods made of a single part, using the method of fracture splitting process to separate the cap from the body of the connecting rod. This technique provided gains as weight reduction and consequently reducing noise and vibration due to the decrease of the oscillating mass from the system. By the literature is estimated cost savings of up to 25%, and better fatigue performance.After the massive introduction of these connecting rods, inexplicable failures have been reported in Brazil and Europe and therefore, several investigations have been conducted trying to discover the reason of this problem. The investigations are mainly concentrated on the fractured surface and its possible effect in reduction of shear load transfer capacity and relaxation of the bolts, under room and service temperature.All hypotheses assumed could not be confirmed. The purpose of this presentation is to show that the root cause comes from the relaxation of the internal thread due to the behavior of high carbon steel microalloyed, when it is subjected to high plastic deformation at the moment of rolling the internal thread. Stress-strain curves were constructed up to the limit of the plastic deformation from specimens machined, removed of connecting road, and the results were compared with the plastic deformation of the internal threads fillets of the connecting rods, leading to identify the origin of failures occurred in the engines, during field service.