Mercury - 300 Hour Maintenance Kit FourStroke EFI Series Marine-Grade Components 8M0151470. Overview. If you run a small Mercury FourStroke regularly, you already know how much those engines become part of your routine. They start before sunrise fishing trips, push you across quiet lakes, and help you ease into shore at the end of a long day. When you reach the 300-hour mark, you want a kit that gives you confidence-something that feels like you're giving your motor the care it deserves. The Mer