Yamaha - Z300 Lower Unit Assembly Lower Unit 6D0-45300-20-8D. Overview. If you've worked around Yamaha Z300 motors, you know they demand a lower unit that can handle serious power without hesitation. This assembly is built to keep everything turning smooth, whether you're pushing hard offshore or running long distances. I've seen what happens when a lower unit starts to wear-noise, vibration, and lost confidence on the water. Put a solid one like this in place, and the whole setup feels tight, responsive, and ready to go.. Key Features. • Heavy-Duty Gearcase Build - Designed to handle high horsepower loads without compromising performance.• Smooth Power Transfer - Delivers consistent propulsion with clean engagement through the drivetrain.• Precision Internal Alignment - Ensures proper gear mesh for reduced vibration and improved handling.• Marine-Ready Construction - Built to perform in both freshwater and saltwater conditions.• Factory-Spec Fitment - Engineered specifically for Yamah