Mercury - Complete Gearcase Bravo Three / Three X / Three XR Series Corrosion-Resistant Alloy 8M0046735. Overview. When you spend enough time on the water, you learn pretty quickly that your sterndrive's gearcase is more than just a mechanical part-it's the heartbeat of the entire propulsion system. When it's healthy, everything feels right. The boat lifts cleanly onto plane, the shifts come through smooth, and the steering stays predictable even in a tight channel. That's the kind of confidence