That means the tach will have a screw like mechanism on the back with numbers like 2, 4, 6, 8 for example. OMC sends a tach pulse. Number of pulses per revolution is where you would set the screw like mechanism too. To keep it easy know what your current tach says the idle RPM on your engine is. Put the new tach in and set that screw to a number. Maybe match the number of cylinders your motor has. Start your engine and see if the new tach is showing the proper idle RPM for your engine. If it does you are good. If it is off from what the old tach said alter the screw on the back of the new tach and try it again.
