Mercury - Water Pressure Gauge Dual Scale White Face 2-1/8" (52 mm) 79-8M0052871. Overview. The Mercury Water Pressure Gauge (79-8M0052871) gives you an instant, accurate look at your engine's cooling system pressure. With its dual-scale display showing both PSI and Bar, you can monitor performance in English or Metric units. The white face, black markings, and glass lens make it easy to read in any light, and it fits a standard 2-1/8" (52 mm) dash opening. Built for SmartCraft-compatible system