In measuring piston ring groove wear, which tool would be least effective?

Using a screwdriver to measure piston ring groove wear would be the least effective choice for several reasons. A screwdriver is not designed for precise measurements; its primary function is to drive screws. It lacks the calibration or gradation necessary for accurately assessing the depth or width of a piston ring groove, which is crucial when determining wear.

In contrast, a new piston ring can be a reference standard to check against the wear in the groove, providing a clear visual indicator of wear compared to the original specifications. A tapered gauge is specifically designed to measure the width and taper of grooves or slots, allowing for accurate assessment of wear. An outside micrometer is a precision measuring tool that can provide exact measurements of groove dimensions, enabling the mechanic to quantify wear effectively. Thus, while the other tools facilitate accurate and relevant measurements of piston ring groove wear, the screwdriver lacks the necessary attributes to serve this purpose effectively.