Engineering change is ubiquitous throughout the product lifecycle to meet new requirements or deal with emerging problems in the product.. Components have a certain capacity to buffer the impact of changes before they pass changes on to other components. These buffers are margins on the components which exceed the current requirements. Typically these margins are designed into a product at the beginning and eroded in the course of the design process or during future upgrades. Fundamental design decisions are being taken based on an understanding of the margins available when considering design alternatives. This paper argues that the knowledge and understanding of these margins is the key to managing engineering changes through the product lifecycle. By tracking key product margins a company can assess when an engineering change could lead to costly knock-on effects.