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Replacing the Coolant Drain Cock on a Yanmar 3GM30 Engine

We recently undertook a coolant change on our marine Diesel engine and at the time we noticed that the plastic threaded tap/petcock on the coolant drain cock assembly had sheared in half. The end knob had disappeared (no doubt into the depths of the bilges!) whilst the other half remained inside the assembly. Whilst it was not causing an obvious problem and could still be screwed in with the assistance of a screwdriver, we decided to replace it.

The coolant drain cock on a Yanmar 3GM30F engine is on the port side of the engine, just aft of the fuel filter.

We ordered a new part from Bottomline Marine, part number 128370-01800, at a cost of £33.89.

We inserted a long section of 6mm drain tube to the existing drain cock to allow us to drain the coolant out to a container.

We lowered the expansion bottle to below the radiator cap height and removed the radiator cap. Using a screwdriver we removed the threaded plastic tap on the existing drain cock which allowed the coolant to drain out. The capacity of coolant in this particular engine is approximately 5.4 litres, albeit not all the coolant will drain out as some will remain in the hot water heater circuit (calorifier). We used a 5 litre container to catch this and probably extracted around 4 litres.

Once the coolant had drained we removed the old drain cock and using a socket on an extension bar installed the new one.

The coolant that we had extracted was poured back in.

And gently ‘pumped’ through some of the hoses to extract as much air as possible.

With the new drain cock in situ and the coolant back in the system, the engine was run to ensure there was no air in the system that would cause an overheat of the engine.

This is a recap of how we replaced the coolant drain cock on our Yanmar engine. It is for guidance only and we do not purport to be experts in this, but hope it may give you some assistance. If in doubt please seek advice from your local dealer or engineer.

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