Mercury - Gear Oil Pressure Gauge 600 PSI Dual Scale Black Face 2-1/8" (52 mm) 79-8M0069429. Overview. The Mercury Gear Oil Pressure Gauge (79-8M0069429) gives you a fast, accurate view of your gearcase pressure so you can spot problems early and protect your drivetrain. It reads up to 600 PSI, with a dual-scale display showing both PSI and Bar, so you can monitor performance in your preferred units. The black face, bold white numerals, and glass lens make it easy to read in any light, and it fi