Modeling of the 1994 Mustang convertible roof mechanism using ‘Variation Simulation Analysis - 3D’ (VSA- 3D)  allowed alternate component datum schemes to be evaluated. Overall roof-to-vehicle interface tolerance targets were set against which component assembly methods were reviewed. Theoretical analysis of these critical interfaces allowed in-depth assessment of manufacturing goals; such as application of robotic welding and ‘no adjust’ installation in the vehicle. The ability to vary assembly methods and component datum schemes and to predict impact on overall system tolerances resulted in reductions of up to 81% in dimensional variation. The final roof design achieved the desired ‘no adjust’ installation in the vehicle.Interface output data from a simulated build of 2000 assemblies is presented in statistical form. Specific component tolerances are identified which contribute significantly to overall interface variation. These features were subsequently targeted for improvement.