Yamaha - Lower Unit Assembly Gearcase & Propulsion System 6H3-45300-05-4D. Overview. When a lower unit starts acting up, you can usually feel it before you even see it-rough shifting, noise, or just not running like it used to. I've dealt with enough of these to know that swapping in a complete assembly is often the cleanest way to get back on the water without chasing multiple issues. It's the part of the motor that actually puts power into motion, so everything has to be solid. Once it's diale