Yamaha - Lower Unit Assembly Gearcase & Propulsion System 6H3-45300-E8-8D. Overview. When a lower unit is solid, you don't think about it-you just run the boat and trust it's going to do its job. But when it starts slipping, grinding, or making noise, it's usually time to stop patching things and replace it outright. I've swapped enough of these to know a complete assembly saves time and headaches compared to chasing internal damage. Once it's on and set up right, the motor feels tight, smooth,