Yamaha - Lower Unit Assembly Gearcase & Propulsion System 6H3-45300-17-8D. Overview. When a lower unit goes, you don't always get much warning-and it usually happens at the worst time. I've replaced enough of these to know that a complete assembly saves a lot of guesswork compared to piecing things back together. It's the backbone of how your motor puts power into the water, so everything has to be right. Swapping in a solid unit like this gets you back to running smooth and confident again.. Ke