Yamaha - Lower Unit Assembly Gearcase Component 6P3-45300-51-8D. Overview. When a lower unit starts going bad, you usually feel it before you see it-noise, vibration, or just not running as smooth as it used to. I've swapped enough of these to know that going with a complete assembly saves a lot of guesswork and time. Everything is already matched and set up the way Yamaha intended. It's the kind of replacement that gets you back on the water with confidence instead of chasing small issues.. Key Features. • Complete Gearcase Assembly - Includes integrated internal components for a full replacement solution.• OEM Matched Components - Built to factory specs for proper alignment and smooth operation.• Efficient Power Transfer - Ensures consistent delivery of engine power to the propeller.• Durable Marine Construction - Designed to handle load, impact, and harsh water conditions.. Engine & Boat Compatibility. Designed for Yamaha outboard engines requiring a complete lower unit replacement.