What can be done to prevent rusting of the plunger if the jack unit is installed in the well hole before elevator completion?

Filling the plunger with oil serves as an effective preventive measure against rusting. Oil creates a protective barrier on the surface of the metal, which helps to reduce the contact between the metal and moisture in the environment. This moisture is a primary factor that contributes to rust formation, so by keeping the metal submerged or coated in oil, you significantly mitigate the risk of oxidation and corrosion.

Other options may have benefits but do not provide the same level of protection as oil does. For instance, sealing the plunger tightly could limit exposure to some degree, but if moisture is trapped inside, it could create conditions for rust. Applying a rust inhibitor may also help, but those products often require specific conditions and may not be as effective without a full protective coating like oil offers. Keeping the plunger dry is essential, but in an environment where moisture is present, simply removing water may not be sufficient to prevent rust entirely without some form of protective coating.

Thus, filling the plunger with oil not only creates a physical barrier but also assists in displacing moisture, which is crucial for preventing rust in the plunger and maintaining the integrity of the jack unit.