What tool is best to use for measuring wear in the top ring groove of a piston after cleaning?

The best tool for measuring wear in the top ring groove of a piston after cleaning is a feeler gauge used in conjunction with a new piston ring. This choice is particularly effective because feeler gauges offer precise measurements that can reveal whether the gap in the ring groove has exceeded acceptable tolerances due to wear.

By using a new piston ring, which has not yet been subjected to wear, the technician can effectively assess how much space is present in the groove. If the gap between the new ring and the groove exceeds the manufacturer's specifications, it indicates wear that could lead to reduced engine performance and efficiency.

In contrast, while a caliper could measure the groove width, it does not account for the specific wear characteristics of the piston ring itself. Measuring with a worn piston ring using a micrometer won't clearly indicate the condition of the groove since the ring's dimensions have already changed due to usage. A dial indicator is typically used for measuring movement or alignment rather than directly assessing the wear fit of a piston ring in its groove.