1. Bottom valve ( not delivered with engine )       2. Cooling water pump       3. Fresh water pump       4. Drain cock for raw water in heat exchanger       5. Heat exchanger

6.  Thermostat  7. Raw water discharge  8. Drain cock for raw water in exhaust manifold  9. Drain cock for fresh water in engine

Here it is sprayed out to provide concentrated cooling of the exhaust passages and the nozzles. The heated
coolant returns via the thermostat 6 to heat exchanger 5 where it is cooled by the raw water circulating in
the cooling coil Raw or sea water is pumped through the heat exchanger by pump 2.

As long as the coolant is cold, the thermostat is closed for return flow to the heat exchanger and it passes
instead through a bypass line direct to the pump suction side. This bypass consists of a special passage in
the cylinder head and block. Flow through the bypass line is slow and the coolant quickly heats up to approx 160 deg F ( 70 deg C ). At this temperature the main passage through the thermostat begins to
open and is fully open by the time the temperature has reached approx 175 deg F ( 80 deg C ), the best
temperature for the engine.
To protect the engine against salt water corrosion and deposits, only fresh water or mixed with a suitable
amount of ethylene should be used in the cooling system


In the three-cylinder engine the raw water is led through the water jackets of the exhaust manifold after it
has passed through the heat exchanger. This cools the manifold and eliminates overheating and fire hazards.
In the four-cylinder engine the raw water is discharged overboard when it leaves the heat exchanger. In

this engine the exhaust manifold is cooled by the fresh water, which is led to the manifold after passing
through the cylinder block and head.
From the manifold it passes via the thermostat to the heat exchanger. The lower part of the four-cylinder
engine’s heat exchanger is arranged for cooling of the engine lubricating oil. This fed by the oil pump in
the crankcase through the heat exchanger and the lubricant cleaner and from there to the various engine
lubrication points.

Zink Electrodes

As a result of the potential difference between the different parts the cooling surface materials, galvanic
can arise during the circulation of salt-water through the cooling system of an engine.
This can happen to all salt-water cooled on the market. In spite of the use of high-quality material, the galvanic current causes removal of material from parts of the cooling system having a higher potential, so
that “corrosion” occurs which weakens the material effected.
In order to prevent such damage occurring on BM engines, zinc electrodes ( zinc plugs ) are fitted into the
cooling systems of marine engines. In this way a difference in potential is caused between the zinc electrode and the cooling surface material which is higher than that existing between the various parts of
the cooling surface. Because of this, any galvanic currents occurring will cause material to be removed from the zinc electrode instead, so that reaction on the cooling surface material is prevented.

                                                                    1053 Engine                                                   1054 Engine

Because of the removal of material from the zinc electrodes which will occur as described above, there
will, after a certain time, be used up, so that replacement must be made.

The zinc electrodes must be removed for inspection every 200 running hours or once a month. If any depositing is found on them, this must be carefully scraped off or brushed off with a wire brush.
If the electrode has been worn away by more than 50% of it’s original size, it should be replaced.

When refitting the electrode it is of the utmost importance that good metallic contact is obtained between
the electrode and the engine contact surface. Ensure that the zinc electrode is fitted into it’s adapter plug
and carefully clean the contact surfaces of the adapter plug, copper gasket and engine material before
carrying out refitting.