Mercury - Water Pressure Gauge Dual Scale Gray Face 2-1/8" (52 mm) 79-8M0065976. Overview. The Mercury Water Pressure Gauge (79-8M0065976) helps you monitor your engine's cooling system at a glance, ensuring proper water flow and safe operating temperatures. The dual-scale display reads in both PSI and Bar, so you can view readings in English or Metric units. It features a gray face, crisp white numerals, and a glass lens for clear visibility in all conditions. Built to fit a standard 2-1/8" (52