Method for the determination of an optimal tolerance interval for the production of a first part having to be assembled with a second part according to interface surfaces and in compliance with an assembly functional requirement (a), said parts being represented by their structural digital mockups, said method comprising the steps consisting in: a. acquiring the nominal theoretical position of a plurality of so-called assembly points on the interface surfaces of the digital mockups (201, 202) of the first and of the second part; b. acquiring the nominal theoretical position of a plurality of so-called structure points distributed over the surfaces of the digital mockups (201, 202) of the first and of the second part; c. acquiring for the pluralities of assembly points and structure points Pi of the digital mockups of the first and of the second part a mechanical restoring stiffness for return to its initial position in the form of a rigidity matrix; d. acquiring the definition of a variable, called the variable of interest, comprising a scalar variable representative of the assembly functional requirement and a cost function giving the value of the variable of interest as a function of the discrepancy of the scalar variable with respect to the nominal of the assembly functional requirement; e. acquiring a set of distortion vectors of the digital mockups of the first and of the second part, said vectors being normal to the surfaces of said mockups; said method being characterized in that it comprises the following steps: f. computing influence coefficients linearly relating each distortion vector to the value of the variable of interest when the digital mockup of a part, distorted by such a vector, is assembled with the digital mockup of the other nominal theoretical part, the assembly being simulated by placing the points of assembly of the two digital mockups in coincidence so as to provide an optimal tolerance of production of the first and of the second part (101, 102) in compliance with the functional assembly requirements (a), g. choosing the positioning devices and the assembly tooling so as to assemble the first and the second part (101, 102) manufactured in accordance with the optimal tolerance of production of the first and of the second part (101, 102), h. assembling the first and the second part (101, 102) manufactured in accordance with the optimal tolerance of production of the first and of the second part (101, 102) using the chosen positioning devices and the chosen assembly tooling. a. acquiring the nominal theoretical position of a plurality of so-called assembly points on the interface surfaces of the digital mockups (201, 202) of the first and of the second part; b. acquiring the nominal theoretical position of a plurality of so-called structure points distributed over the surfaces of the digital mockups (201, 202) of the first and of the second part; c. acquiring for the pluralities of assembly points and structure points Pi of the digital mockups of the first and of the second part a mechanical restoring stiffness for return to its initial position in the form of a rigidity matrix; d. acquiring the definition of a variable, called the variable of interest, comprising a scalar variable representative of the assembly functional requirement and a cost function giving the value of the variable of interest as a function of the discrepancy of the scalar variable with respect to the nominal of the assembly functional requirement; e. acquiring a set of distortion vectors of the digital mockups of the first and of the second part, said vectors being normal to the surfaces of said mockups; f. computing influence coefficients linearly relating each distortion vector to the value of the variable of interest when the digital mockup of a part, distorted by such a vector, is assembled with the digital mockup of the other nominal theoretical part, the assembly being simulated by placing the points of assembly of the two digital mockups in coincidence so as to provide an optimal tolerance of production of the first and of the second part (101, 102) in compliance with the functional assembly requirements (a), g. choosing the positioning devices and the assembly tooling so as to assemble the first and the second part (101, 102) manufactured in accordance with the optimal tolerance of production of the first and of the second part (101, 102), h. assembling the first and the second part (101, 102) manufactured in accordance with the optimal tolerance of production of the first and of the second part (101, 102) using the chosen positioning devices and the chosen assembly tooling.