The paper presents a computing method for the analysis of the precision of the six links Watt and Stephenson mechanisms due to the clearances from the kinematics joints. The mathematic model is developed on the basis of the Monte-Carlo method, used to simulate the positions of the direction of the contact between the elements which form the joints. The numeric examples lead to the conclusion regarding the establishment of the clearances from the joints in such a way that the precision realized by the mechanism to fit in the tolerance imposed by the design.