Which tool measures clearance between a surface plate and a narrow surface being checked for flatness?

The tool that effectively measures clearance between a surface plate and a narrow surface being checked for flatness is the thickness gauge. This instrument is specifically designed to measure the thickness of materials, which can directly relate to assessing flatness by determining if there is any gap or clearance between the surface being measured and a reference plane, such as a surface plate.

Thickness gauges can come in various forms, including digital, mechanical, or feeler gauges, which can fit into narrow spaces to provide accurate readings. When evaluating flatness, the ability to measure small clearances is essential, making this tool particularly suitable for this task.

Other instruments mentioned, like depth gauges and dial indicators, have different primary functions. Depth gauges are primarily used to measure how deep a feature is, while dial indicators measure variations in height or distance from a reference point rather than specifically measuring the gap between surfaces. Calipers, though versatile for measuring dimensions like length or width, are not specifically optimized for measuring clearance in flatness checks. Therefore, the thickness gauge is the most appropriate tool for this particular measurement scenario.