<p>The paper integrates the design of constant torque generators, optimal mechanism synthesis, and position error analysis. The approximate constant torque generator composed of a four-bar linkage and a spring is considered. The optimal dimensions of the mechanism were determined using a synthesis heuristic method. Recently developed mechanism synthesis methods make it possible to determine geometries of the linkages that can realize the required performance with very high accuracy provided that a solution for a prescribed problem exists. However, manufacturing problems are frequently neglected, and, as a consequence, the imposed accuracy cannot be achieved when errors due to tolerances, link deformations, assembly errors, and friction are taken into account. Therefore, the objective of the paper is to study the relation between the geometrical tolerances and the accuracy of the prescribed torque generation. The worst-case error is determined using the direct linearization method for prescribed tolerance grades. Three different four-bar linkages are taken for illustrative purposes, and the general conclusions are put forward on their bases.</p>