Mercury - Lower Unit Gear Lube Drain and Fill Hole Screw & Seal, Magnetic Gearcase Hardware Magnetic Steel 8M0058389. Overview. There's a certain kind of comfort that comes from knowing the simplest parts on your boat are the ones you never need to think about. Anyone who's been around the water long enough knows it's often the little things - a seal, a plug, a gasket - that keep a good day from turning into a long tow back to the dock. That's why a dependable lower unit drain and fill screw mat