Yamaha - Lower Unit Assembly Complete Drivetrain 6K5-45300-13-4D. Overview. The Yamaha Lower Unit Assembly 6K5-45300-13-4D is what really puts your engine's power to work in the water. When a lower unit starts going bad, you'll hear it first-then feel it through vibration or loss of push. At that point, chasing individual parts can turn into a headache, so dropping in a complete unit like this just makes sense. It's the kind of fix that gets you back on the water with confidence instead of second-guessing every run.. Key Features. • Complete Gearcase Assembly - Includes core internal components for full lower unit operation.. • Smooth Power Transfer - Delivers engine output efficiently to the propeller.. • Corrosion-Resistant Build - Designed to handle saltwater and freshwater exposure.. • OEM Fit and Alignment - Matches Yamaha specifications for proper installation and performance.. • Superseded Replacement Design - Updated by a newer part number for continued support.. Compatibility.