What material is primarily used for tri-metal engine bearings?

Tri-metal engine bearings are designed for high performance and durability and typically consist of a layered material structure. The primary composition involves a Babbitt alloy made up of tin and other metals, which provides a soft, wear-resistant surface that is essential for the bearing's operation. This Babbitt layer is often bonded to a copper lead substrate, which enhances the bearing's overall strength and load-bearing capacity.

This layering is critical as it allows the bearing to maintain a lubricating film under high loads and speeds, reducing friction and wear on the engine components. The copper/lead backing serves to provide rigidity and support while the Babbitt surface takes on the brunt of wear.

In contrast, while aluminum is often used for other types of bearings due to its lightweight properties, it does not offer the same wear resistance or durability as a tri-metal design. Bronze, although historically used for bearings, lacks the specific properties that tri-metal combinations provide. Steel, on its own, is not suitable for bearings due to issues with lubrication and wear resistance, making it ineffective in applications requiring high-performance bearings. Therefore, C is correct as it accurately identifies the composition commonly used in tri-metal bearings, combining both durability and performance characteristics.