Yamaha - Lower Unit Assembly Gearcase & Propulsion System 6H3-45300-E8-4D. Overview. When a lower unit starts giving trouble, it's rarely something small, and it usually shows up when you're already out on the water. I've replaced enough of these to know that going with a complete assembly saves time and headaches compared to chasing internal damage. This is the part that takes all the engine's power and turns it into movement, so it has to be right. Once it's installed properly, the motor feels