Plunger surfaces should be cleaned with ______ and coated with ______ before being lowered into the cylinder.

Plunger surfaces should indeed be cleaned with emery cloth and coated with hydraulic oil before being lowered into the cylinder. Using emery cloth helps ensure that the plunger is free from debris, scratches, or any imperfections that could affect its performance within the cylinder. This step is crucial for maintaining the smooth operation of the jack and preventing any potential jamming or scoring of the cylinder walls.

Coating the cleaned plunger with hydraulic oil is essential for lubrication. The oil not only provides a protective layer that reduces friction as the plunger moves in and out of the cylinder but also helps to prevent corrosion. Proper lubrication is vital for the longevity and reliability of hydraulic jacks, ensuring optimal performance during operation.

The other options do not provide the appropriate means of achieving the desired results. For example, water can cause rust and is not a suitable lubricant, while sandpaper may be too abrasive and could damage the plunger surface. Detergent may not offer lubrication properties and could also leave residues that interfere with the movement within the cylinder. Therefore, using emery cloth and hydraulic oil is the established best practice for preparing plunger surfaces in hydraulic systems.