Mercury - Impeller Key Cooling System Component Stainless Steel 8M0032831. Overview. There are a few pieces on an outboard that don't look like much at first glance, yet you realize just how vital they are the moment something goes wrong. The impeller key is one of those quiet heroes. It's a small part, sure, but when you're pushing across a glassy bay at sunrise or easing into a tight slip after a long day on the water, you want every single component in your cooling system doing its job withou