If your engine has a
heat exchanger it may not have any anodes and since they are not mentioned in the
service manual this is most probably the case. You cannot just drop them in because the anode has to have a good
electrical connection to the metal you are trying to protect.
Modern engines tend to have alloy
parts exposed to the
cooling fluid and if you run them on just water instead of the recommended cooling fluid you could have serious electrolytic
corrosion problems in that the alloy
parts will act as an anode to protect any other exposed metal parts particularly cast iron or
steel engine blocks.
Many of the
marine conversions now prevalent have the heat exchanger incorporated into the water cooled
exhaust manifold and these are cast from alloy and are particularly prone to
corrosion if you don't run the correct cooling fluid in the engine side of the circuit.
It is probably more important to use the correct cooling fluid than to install an anode although I tend to like anodes and have lots of them on my engine.