Which type of wear failure is caused by lubrication film that is too thin for proper separation?

The correct choice highlights adhesive wear as a type of failure that occurs when there is an inadequate lubrication film thickness. When the lubrication film is too thin, it cannot effectively separate the moving surfaces of the engine components. This leads to direct metallic contact, which causes materials to adhere to one another. As the surfaces attempt to slide past each other, some of the material can transfer from one surface to another, resulting in wear. This process continues as long as the lubrication remains insufficient, ultimately leading to significant damage and failure of the components involved.

Understanding adhesive wear is critical for maintenance and operation of four-stroke engines because it emphasizes the importance of proper lubrication and the need to monitor oil levels and quality to ensure a sufficient lubrication film is present.