Google brings up loads of suppliers/manufacturers

Not tried, but

https://www.coxandturner.co.uk/piston-rings/

If oil is getting into the cylinders, either from leakage past piston rings or from valve guide seals it should be burning it with the diesel fuel and coming out as blue smoke, do you have any blue smoke in the exhaust gas under load?

Is this a two-stroke diesel or conventional four stroke?

Is this a direct sea-water cooled engine or indirect cooled via a heat exchanger?

And what is the engine’s coolant temperature on load?

What engine oil spec is it running?

An engine model number/hp/no of cylinders would help.

On 18 March 2025 at 20:00 Howard Lewis Said:

Oil carry over is often caused by glazed bores, usually caused by running at high speed, light load.

Short of stripping and deglazing the bores, it may be possible to cure by prolonged high load running.

Howard

Possible but this usually is confined to new or newly rebuilt genset diesel power units, where they are run up weekly/fortnightly/monthly without being connected to a load bank. It’s reportedly caused by anti-wear additives in the lube oil building up on the cylinder walls and not being scrubbed off by the ring pack if there is not enough combustion pressure on light load to expand the rings out against the cylinder wall.

I would have thought unlikely in this case as it sounds as if the engine in question is old and hasn’t been rebuilt.

Can happen to petrol engines too.

“It uses direct seawater cooling at about 36°C, and we now run it on 100% pure mineral oil”

From my experience with a manufacturer, running direct sea water cooled marine diesel engines at a low 36 deg C will likely result in piston rings sticking in the piston grooves due to combustion residue fouling.

That is is of course unless you have original manufacturers data detailing that 36 deg C is the correct engine block cooling temperature to run the engine at?

How is this temperature regulated? By using a built in thermostat, or by engine room operatives restricting the flow by valves?

Normally, a pure mineral oil, without detergent additives, is not correct for running any diesel engine at the low block temperatures required when the engine is direct sea water cooled, that’s normally 50 to 60 deg C.- not in my experience 36.deg C.

You need a high detergent content oil to keep the rings free with a sea water cooled engine, not a straight mineral oil.

Contact your oil supplier to find out what they can supply to suit the engine, I hesitate to recommend any grade/viscosity of a modern oil, but if it was a reasonably recent engine I would be looking at a minimum API CD spec oil which no longer exists, so again, contact your oil supplier.