Yamaha - Lower Unit Assembly Gearcase Component 6P2-45300-51-8D. Overview. On an F250, the lower unit is where you really feel the condition of the motor once you're underway. I've worked on boats where everything looked fine up top, but the gearcase was holding it back with noise or rough engagement. Swapping in a solid assembly like this brings back that smooth, confident feel when you shift and throttle up. It's the kind of fix that makes the whole boat run the way it should again.. Key Features. • Complete Gearcase Assembly - Provides a full replacement with matched internal components.• Efficient Power Transfer - Delivers smooth and consistent output to the propeller.• OEM Precision Fit - Built to Yamaha specifications for proper alignment and installation.• Corrosion-Resistant Construction - Designed to withstand harsh marine environments.. Engine & Boat Compatibility. Designed for Yamaha F series outboard engines, including high-horsepower four stroke models. Commonly used acros