Yamaha - Lower Unit Assembly Gearcase Component 6P2-45300-31-8D. Overview. On an F250, the lower unit is doing serious work every time you push the throttle. I've seen these motors run perfectly up top but feel off down low when the gearcase starts wearing out. Swapping in a complete assembly like this brings everything back to how it should feel-smooth, quiet, and solid under load. It's the kind of replacement that takes the guesswork out and gets you back on the water with confidence.. Key Features. • Complete Gearcase Assembly - Provides a full replacement with properly matched internal components.• Efficient Power Transfer - Delivers smooth and consistent output from engine to propeller.• OEM Precision Fit - Built to Yamaha specifications for proper alignment and performance.• Corrosion-Resistant Construction - Designed to handle saltwater and harsh marine environments.. Engine & Boat Compatibility. Designed for Yamaha F series outboard engines, including high-horsepower four strok