In order to integrate tolerance synthesis for manufacturing dimensions in CAPP systems, an automated process plan simulation module has previously been developed. This module permits the verification of the manufacturing pre-projects and the optimization of the manufacturing dimensions. The module used an optimization procedure which was realized through an equal distribution of the excess tolerance which is that of the design functional requirement. However in reality, this distribution should take into account the stochastic aspects of the dispersions and the complexity or cost of the manufacturing dimensions. In the present work, in order to overcome these drawbacks, a statistical and cost based tolerance synthesis model has been developed. It is shown that this model performs an objective and global distribution of the residual (excess) tolerances of the design functional dimensions on all the manufacturing dimensions of the process plan. The dispersions method is used in the modelling process to determine the variables of the objective function and to automatically extract the manufacturing tolerance chains. These chains are then used to construct the functional constraints of the optimization model. The whole process has been programmed as an automated tolerancing tool for manufacture and validated for several examples.