Yamaha - Lower Unit Assembly Gearcase & Propulsion System 6H3-45300-19-4D. Overview. A bad lower unit will take a solid-running motor and turn it into a headache fast. I've seen guys chase small fixes when the real issue was deeper in the gearcase, and at that point, a full replacement just makes more sense. This assembly takes the guesswork out and gets everything back in line where it should be. Once it's installed right, you'll feel the difference immediately when you put it in gear and get o