The oil ring should be installed in which groove?

The oil ring should be installed in the bottom groove of the piston. This design is intentional because the oil ring is responsible for controlling the amount of oil that lubricates the cylinder walls and helps to maintain proper compression in the combustion chamber. By placing the oil ring in the bottom groove, it ensures that it can effectively scrape excess oil from the cylinder walls back into the oil sump, thereby reducing oil consumption and minimizing the risk of oil entering the combustion chamber, which could lead to excessive smoke and undesirable emissions.

Additionally, installing the oil ring in this position allows it to better manage oil distribution throughout the engine, ensuring sufficient lubrication to moving parts without causing excessive oil build-up that could interfere with combustion or lead to engine performance issues.