Mercury - Water Pressure Gauge Dual Scale Black Face 2-1/8" (52 mm) 79-8M0052850. Overview. The Mercury Water Pressure Gauge (79-8M0052850) gives you a quick, accurate look at your engine's cooling system performance. It features a dual-scale face displaying pressure in both PSI and Bar, so you can read it easily in English or Metric units. With its black face, white markings, and glass lens, it fits right in with Mercury's OEM black-face gauge family. It installs into a standard 2-1/8" (52 mm)