Mercury - Replacement Thermostat 2-Stroke 30-60HP Outboards Quicksilver 850055001. Overview. There's something reassuring about knowing your engine is running right where it should - not too hot, not too cold, just steady and strong. The Mercury Quicksilver 850055001 Replacement Thermostat keeps your outboard in that perfect temperature zone, so you can focus on the water, not your gauges. Whether you're pushing through the backwater shallows or easing down a quiet river at sunrise, this part qu