Certain mechanical assemblies (transmission boxes, cylinder head valve actuation for internal combustion engines, fuel injection systems and so on) require strict control of dimensional stack up tolerances of their assembled components. This is necessary in order to provide working components that, once assembled, operate in relative silence while at the same time provide a long durable life. That objective is not economically achievable by match production of the individual components (with their tight tolerances) to achieve the cumulative tolerance of the assembly. For this reason one of the following criteria is usually applied:

• A classified shim is inserted between the components of the assembly. Specific sizes of the matched components are measured to calculate the shim class required to achieve the final assembly tolerances within the established limits.
• The components for assembly are produced with an economically viable tolerance and divided within very specific classes. Parts of corresponding classes are matched together to complete the final assembly.