Coral 2 Cylinder Head Temperature Gauge 60-120 F with SenderPerimeter-lighted Black dial with stylish White and Blue graphics, Black aluminum bezel with burnished leading edge, contoured White pointer, and flat glass lens.A temperature gauge operates by sending a low amperage current through the gauges meter to the ground via a sending unit with variable resistance. The resistance of the sending units increases or decreases with the changes in pressure, temperature, etc.2 - Hole Size 2.0625 53mm