Cummins Diesel Engine School Practice Test

Main bearing shells use a tri-metal structure: a soft, conforming bearing surface for embedability and smooth rotation, a copper-based interlayer for strength and heat transfer, and a backing metal that provides rigidity and support. In this exam context, the trio includes lead as the soft bearing surface, a copper-based interlayer (represented by the Gold-66 alloy), and the backing metal. Aluminum is used as the backing material in many modern bearing shells, completing the three-part stack and giving the bearing its structural integrity without adding excessive weight. The other options mix in metals that aren’t part of the standard bearing-stack trio in this context, so aluminum is the metal that completes the set.