Form, Fit & Function – Additional Considerations

Form, Fit & Function is a change and configuration management principle that helps to determine if a component or assembly can be revised, i.e. if it can be changed by increasing the revision of the component or assembly, or if a new component or assembly has to be created and given a new part number when it is changed.

The previous article explored how the principle of Form, Fit & Function can be used to help determine whether to revise a part or create a new part and explained in detail the meaning of the three criteria Form, Fit and Function.

I have since been asked a few times if Fit doesn’t mean fitting in the place of the previous component, meaning “connect to, mate with, or join to another feature or part within an assembly”, as defined in the Wikipedia article about “Form, fit and function” (,_fit_and_function).

While this is certainly true – a new revision has to fully “connect to, mate with, or join to another feature or part within an assembly”, this definition is not complete. As stated in my previous post, fit is not only limited to technical aspects and considerations, but also regulatory and other ones. Fit is the entire suitability, qualification or fitness of a component or assembly to replace a preceding component or assembly, not just the geometric fit.

So for example changing a material of a component may be technically possible and may not affect the geometric fit of the component, but it may not be legal to use the new material in all countries where the component is currently in use. Consequently not all existing components would be replaceable with a component made of the new material, and hence the existing component could not be revised but a new component would have to be created and assigned a new part number.

The important take away from this expanded definition of Fit is that deciding whether to revise a component or assembly or to create a new one may not solely be possible in the engineering department. Engineering may not always know where components are used in the field and what the regulatory requirements and rules are where the components are used. A cross-functional change board that includes sales, marketing, customer service, quality, regulatory affairs, manufacturing, etc that is involved in change impact analyses is hence often highly advisable and beneficial to avoid costly mistakes.