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
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.