Mercury - Upper Driveshaft Housing Bravo One XR Integrated Transom Corrosion-Resistant Marine Alloy 863836A73. Overview. There's a certain kind of confidence that settles in when your boat responds exactly the way you expect it to. When the power comes on smooth, the drive feels planted, and the whole rig feels like it's working with you-not against you. That peace of mind starts with the parts you choose, especially the ones buried deep in the heart of the sterndrive. This Mercury Marine part d