Working on my '84 50hp mariner replacing the water jacket gasket. I discovered, what I thought was a broken off bolt next to the top spark plug, is really a tapped hole to accept some sort of a sensor? What type of sensor may have been there, as I can not find any informtion in my repair manual or on any parts list.

Also, I would like to tach out this motor and understand that the control lever box has a socket to accept a harness? Were can I get a harness and tach for this motor?