Euro Black 4 Tachometer with Hourmeter 6,000 RPM Gas Inboard . 4 Hole size 3.375 85mm . Electronic tachometers work by counting pulses generated by the ingition system, alternator, tach signal generator or magnetic pickup sender. The tach is hooked up to 12VDC, Ground and one of hte signal sources listed above. By selecting the right tach and setting the switch on the back to the correct postion you let the tachometer know how many pulses are sent per each engine revolution. Benefits Wide range of scales. Multiple signal sources. Rugged, Reliable construction. 7 character LCD display. 0-999999.9 Hours. Updates in .1 increments 4 Hole size 3.375 85mm . Electronic tachometers work by counting pulses generated by the ingit By selecting the right tach and setting the switch on the back to the correct postion you let the ta Wide range of scales. Multiple signal sources Rugged, Reliable construction. 7 character LCD display 0-999999.9 Hours Updates in .1 increments