Mercury - Standard Rotation Gearcase MerCruiser R/MR/Alpha One Low-Copper Alloy & Tri-Paint Finish 8951A43. Overview. If you've spent enough time piloting a sterndrive boat, you know how much the lower unit shapes the feel of every trip. The way the boat lifts, how it holds in a turn, the steady pull when you push up onto plane-all of it starts in the gearcase. When the gearcase is strong and healthy, everything about the boat feels tighter, smoother, and more predictable. This Mercury Marine pa