A bore gauge is primarily used to measure which aspect of cylinders?

A bore gauge is a precision measuring instrument specifically designed to measure the internal diameter of a cylindrical object, such as an engine cylinder. When used in cylinder measurement, it is essential for determining any taper that may exist along the length of the cylinder. Taper refers to a condition where the diameter of the cylinder is not uniform, meaning it can be wider at one end compared to the other. This can lead to increased wear on the piston and potential engine performance issues.

Identifying taper is crucial for engine performance and longevity because uneven wear can affect the seal between the piston and the cylinder wall, reduce compression, and ultimately lead to poor engine operation. In contrast, the other options do not directly pertain to the primary function of a bore gauge. While depth measurement is important, it is typically done with different tools, as is the measurement of wall width. The diameter of the piston is also not the primary focus of a bore gauge, which is more concerned with the internal characteristics of the cylinder itself.