Recently acquired a 1969 Mercury 65 HP outboard and would like to add
a tachometer. It's the 650 model with Thunderbolt ignition and a
label under the cowl warns me about using any tachs not designed for
the motor. The control box up front does have a 3-prong receptacle
for a tach, but I haven't had much luck finding one to fit.

Do I really risk hurting the motor by using a non-Merc tach? Could I
just go inside the control box and hook up a modern aftermarket tach?

Thanks in advance.

Dave