Abstract: The physical and chemical inspection and failure cause analysis of the torn speed nuts during the assembly of the bolts on the automobile assembly line were carried out. The results show that the failure of the speed nut was a one-time overload tearing due to the large external force. Recovering and analyzing of the assembly site, it was found that the relative position between the speed nut and the two sheet metal parts connected thereto was mismatched. As a result, the speed nut overload tearing failure occured due to misalignment during assembly.