Yamaha - Lower Unit Assembly Gearcase Component 6D9-45300-15-8D. Overview. The Yamaha Lower Unit Assembly 6D9-45300-15-8D is what keeps your engine's power translating into smooth, reliable movement on the water. When a lower unit starts to wear or fail, you'll feel it immediately-grinding noise, vibration, or reduced performance. This assembly handles the full transfer of power from the engine to the propeller, so everything depends on it being solid and properly aligned. If your motor isn't running like it should, replacing the lower unit is often the most effective fix.. Key Features. • Efficient Power Transfer - Delivers engine output smoothly to the propeller for consistent performance.. • OEM Fit Precision - Built to Yamaha specifications for proper alignment and easy installation.. • Complete Gearcase Assembly - Pre-assembled unit for straightforward replacement and dependable operation.. • Corrosion-Resistant Construction - Designed to withstand harsh freshwater and saltwater e