Bolted joints are largely used in machines and equipment, allowing joining together parts from different materials, easy access for mounting and maintenance and the possibility to change smaller damaged parts instead of a larger assembly. However, if the pre-tensioning in the joint is not sufficient, the assembly would not function properly, and if the pre-tensioning is too high, the parts could be damaged. Therefore, controlling the pretension is essential. In this paper, a method to determine the correspondence between the torque applied by a dynamometric key and the pre-tension is presented. Also, tests for determining the friction coefficients in the thread and in the contact between the nut and washer were performed, to underline the differences between different materials and coatings, and the influence of self-locking rings in nuts.