1978 Mercury 140HP Tachometer options? "Mercury only"

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Aug 3, 2010
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I have a 1978 140hp Mercury outboard with a non-working tach. The tach doesn't appear to be a mercury product. It says somewhere on the outboard to use "mercury only" tachometers. Is that a "hard" rule? If I need to use mercury. Will this part number work?
15110A 1
 
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