Coral 4 Tachometer 7000 RPM All Outboard Perimeter-lighted Black dial with stylish White and Blue graphics, Black aluminum bezel with burnished leading edge, contoured White pointer, and flat glass lens.Electronic tachometers work by counting pulses generated by the ignition system, alternator, tachometer signal generator or magnetic pickup sender. The tachometer is hooked up to 12 VDC, Ground and one of the signal sources.4- Hole Size 3.375 85mmSpecificationsScratch Resistant LensBlack Aluminum