Mercury - Quicksilver Thermostat Cooling System Components Marine-Grade Construction 8M6010300. Overview. There's a quiet confidence that comes from knowing your outboard is running at the exact temperature it was designed for. When the water's calm and the morning air is cool, or when you're pushing across choppy afternoon swells, you want your engine to stay steady and predictable. That's exactly what the Quicksilver 8M6010300 thermostat delivers. It keeps your Yamaha 75-90 horsepower four-str