When measuring clearance in an assembly, which tool is most appropriate?

The most suitable tool for measuring clearance in an assembly is a thickness gauge. Thickness gauges are specifically designed to measure the gap or space between two parts in an assembly, which is essential for ensuring proper fit and functionality. They can provide precise measurements of small gaps, allowing for the assessment of tolerances in mechanical design.

While calipers and rulers can measure dimensions, they are not as optimized for gauging clearance, especially in tighter spaces where precision is crucial. A caliper can measure external and internal dimensions but may be cumbersome in situations requiring the assessment of a small clearance between parts. Rulers also lack the granularity and specialized design needed for precise clearance measurements. A protractor, on the other hand, is used for measuring angles, making it irrelevant to the task of measuring clearance.

Thus, for measuring the clearance effectively and accurately within an assembly, a thickness gauge is the right choice.