In design sizing, should parts be made larger or smaller?

• A. Larger
• B. Smaller
• C. Equal
• D. Unknown

Answer: (B)

Design sizing is all about ensuring a reliable fit across manufacturing variation. By sizing parts slightly smaller than their mating features, you introduce deliberate clearance that accommodates tolerance stack-up, tool wear, and measurement variation. This makes assembly smooth and reduces the risk of binding or interference. If parts were made larger, or kept equal to nominal, the inevitable deviations from exact sizes could push the fit into interference, making assembly difficult or impossible. For example, a shaft going into a bore benefits from a small clearance so it slides in easily even when both parts vary a bit in size. So smaller sizing is the practical choice to guarantee a dependable fit.