A bore gauge is used to measure:

The function of a bore gauge is primarily to measure the internal diameter (bore) of a cylindrical object, like an engine cylinder, ensuring it remains within specified tolerances. The most common use of a bore gauge involves checking for taper, which refers to a variation in the cylinder's diameter along its length, indicating wear or improper machining. This is essential in assessing the condition of a cylinder before making decisions about rebuilding or replacing engine components.

While a bore gauge can certainly aid in assessing the taper, it is also capable of measuring other dimensions critical to engine performance. For instance, it can be utilized to measure piston bore, which is the diameter of the cylinder where the piston travels. Additionally, it helps assess the stroke length indirectly by allowing mechanics to determine if the bore dimensions are within acceptable limits for the engine's operation.

Piston ring end gap is another important measurement, but it typically requires different tools like feeler gauges rather than a bore gauge.

In light of this, the correct answer indicates that the bore gauge is an essential tool for measuring taper specifically, but its capabilities extend beyond this single function to include other important measurements related to engine repair.