Dimensional Management (DM) is a methodology to predict and control the impact of variation on assembly from, fit, and function. Application of Dimensional Management tools and other modeling and simulation techniques are combined in a process called 3D Re-Engineering for application to existing production designs. Analytical techniques for predicting the impact of variation on assembly fit, and corresponding methods for controlling variation are presented, as used in a production environment for root cause corrective action on existing assembly fit problems.Assembly variation analysis is typically performed early in the product development phases, by coordinating datums, assembly sequences, assembly methods, and detail part tolerances across the product development team. The 3D Re-Engineering process employs variation analysis techniques, along with other 3D modeling and simulation tools, to determine root causes of assembly fit problems, and to provide improved product and process definition Analytical results show ability to meet specification given a process's capability (Monte Carlo simulations), and contributors to variation which are causing the assembly fit problems (Pareto ranking), which also may be candidates for Key Characteristics, along with methods used for product build such as integrated Quality Plans.3D Re-Engineering techniques have been applied to several key production programs which have experienced recurring fit problems, and have resulted in a comprehensive solution to solving such problems. Solutions range from engineering nominal changes, coordinating tooling datums, implementing process capability, improved work instructions, and others. With the application of 3D Re-Engineering techniques, reductions in assembly rework, reduced engineering, planning, and tooling changes, and reduced assembly cycle time can be demonstrated on existing production programs.