Chesapeake White SS 4 Tachometer - 6 000 RPM (Gas - Inboard I/O)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 the 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 scalesMultiple signal sourcesRugged Reliable construction