Chesapeake Black SS 4" Tachometer with Hourmeter - 6,000 RPM (Gas - Inboard)Perimeter-lighted Black dial with stylish Blue and White graphics, polished stainless steel bezel, contoured white pointer and domed glass lens.Electronic tachometers work by counting pulses generated by the ignition system, alternator, tach signal generator or magnetic pickup sender. The tach is hooked up to +12VDC, Ground and one of the signal sources listed.By selecting the right tach and setting the switch on the back to the correct position you let the tachometer know how many pulses are sent per each engine revolution.4" - Hole size: 3.375" (85mm)Specifications:. Anti-scratch glass lens. 12 VDC negative ground. Can be adapted for 24 VDC systems. Technical Specifications:. 7 character LCD display. 0-999999.9 Hours. Updates in.1 increments. Factory P/N TC9127