Tips for draining engine oil on a small marine diesel engine.

The oil in a diesel marine engine is used for lubricating the internal components, such as crank bearings.

Without lubrication there would be a high level of Friction generated inside the engine.

This friction would be due to the metal components ‘rubbing’ together, in other words ‘metal on metal’.

Such high levels of engine friction would result in damage to the engine components and rapid engine failure.

The  job of the oil inside a marine diesel engine is to create a ‘cushion’ between the metal components, to minimise friction.

An example of engine components that do this, is the ‘big end bearings’.

The Big End Bearings connect the pistons (via a connecting rod) to the Crankshaft.

When the engine is started, the oil pump starts to pump oil around the engine via  a series of small holes.

These holes supply pressurised oil to the engine components, such as the Big End Bearings.

The oil being fed to the bearings creates a thin cushion of oil between the bearing faces and the crankshaft, therefore ensuring that engine friction is minimised.

Draining the Oil

The tips and advice I am giving on this page are aimed at small diesel engines, such as three & four cylinder diesels from manufacturers such as Volvo & Yanmar.

Firstly, as with all combustion engines the engine should be warmed up to normal operating pressure.

This is because the oil will drain easier when warmed up.

Once the engine is warmed up to normal operating temperature, it should be shut down, and the appropriate safety shutdown / lock off procedures followed (refer to engine handbook / company policies & operating safety rules).

Use a suitable oil drain tray, and place underneath the engine, just under the ‘sump plug’.

The sump, or drain plug can now be loosened by turning in an anti-clockwise direction.

At this stage it is important to double check that your receptacle for draining the oil into is large enough (before you totally remove the sump plug).

Information about how much engine oil is held inside the engine, is available in the manufacturers handbook.

Once the sump plug has been removed the warm engine oil (assuming you have warmed the engine up to normal operating temperature) will start to flow out.

Refilling with fresh engine oil

Once all the oil has been drained out from the sump plug hole, then you are ready to start refilling.

Before you do, it is normal practice to replace the Oil Filter at the same time as you carry out an oil change.

Most oil filters on small marine diesels are screw on (though not all), and therefore can be quickly and easily replaced.

Using a suitable oil filter tool to grip the old oil filter, turn  anti-clockwise. The filter should then come loose, and be able to be spun off.

If the oil filter does not easily come off, due to previous over-tightening, then elsewhere on this site, I will explain how to overcome this issue.

Once the old oil filter is off, fit a new oil filter by carefully screwing it on in a clockwise direction.

Before fitting, first fill the new oil filter with fresh engine oil. This helps with quick recirculation of oil when the engine is started, as oil does not need to be pumped into the filter to fill it.

Before you start to fit the new filter, smear a light coating of engine oil around the filter rubber seal. Moistening the rubber seal helps make it seal, and prevent leaks.

Tighten the new oil filter finger tight (see manufacturers instructions). The filter should not be tightened hard on using tools, as this can damage the filter, and lead to leaks.

It also can make it hard to remove during the next oil filter change.

Once you have fitted a new Oil Filter, and replaced the drain plug you are ready to refill the engine with fresh oil.

To prevent possible leaks, it is advisable to either replace or heat up the copper drain (sump) plug washer, to make it malleable again.

If you reuse the same copper drain plug washer, you should suspend it from a suitable piece of  metal, and heat up the washer using a plumbers blow torch, until ruby red.

This will make it malleable again, and help prevent leaks from the drain hole.

Fit the washer onto the drain (sump) plug, and refit by screwing in a clockwise direction.

The drain plug should not be overtightened, otherwise you can damage the thread on the engine sump, which is what the drain plug screws into.

The manufacturers service instructions will contain information as to the correct torque settings.

The torque settings tell you how tight to tighten up the sump plug.

To tighten to the correct torque, a ‘torque wrench’ should be used. These are adjustable to set the correct torque.

Once the new filter is correctly fitted,  and the the sump plug screwed back in, to the correct torque, the new oil can be added to the engine.

It is important for the longevity and performance of your marine diesel engine, that the correct grade of oil is used.

Check with the engine manufacturers servicing handbook, for the correct grade and quantity of engine oil.

Once the new oil has been added crank the engine over to get up the oil pressure. This is ideally done without actaully starting the engine (see manufacturers servicing notes).

Once the engine is running, check for any leaks, and remedy if necessary.

Finally, record the date and engine mileage / hours at the time of the service, so that the next routine service can be carried out at the correct interval.



(c) Craig Miles 2017, including photographs & content. All rights reserved.

Published by Craig Miles

Craig Miles

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