Mercury - Complete Gearcase Bravo One / One X Series Corrosion-Resistant Alloy 8866A63. Overview. There's a moment every boater knows - that quiet pause right before you throttle up, waiting for the boat to answer. When everything is working the way it should, the response is instant: smooth lift, clean power, and a feeling that the whole boat is tuned to your touch. That kind of confidence starts deep inside the drive system. The Mercury Marine part number 8866A63 Complete Gearcase is built for