I stupidly ran my 5hp Mariner until it stopped after pulling off the fuel hose. Should have stopped it normally. Got away with it for years, but finally it caught up with me, and the piston has seized in storage.

There is a chemical/rust dissolver called "Engine Release" made in Canada, just for this application. Does anyone know where or if it is available in Sydney, or an alternative similar magic potion?

Running the fuel dry wont seize the piston on an outboard. Seizing due to running out of fuel with a 2 stroke can occur if the motor is still being turned over at high speed and/or for a long duration after the fuel is shut off, but this situation is extreme / almost mythical and near impossible with an outboard and often its not the lack of fuel lubrication that causes the high speed seize with a 2 stroke ,but more so shock cooling.(more likely with an air cooled engine).

As the fuel runs out on a heavy loaded engine, the fuel mixture (fuel/air) can go lean and can result with a hole through the piston.

The book says stop it when it starts to run rough, after disconecting the fuel line, so that some fuel and oil is left in the engine. It was sitting for more than a year, so this time I guess it needed that little bit of oil in the cylinder. Apparently the problem is common in countries where they lay the boats up for winter.

there is know way this eng seized from running the fuel out check for water from exhaust plate or over heating, the oil does not burn away easy maybe oil mix is incorrect, put some diesel down plug holes and work back and forth lightly,if it frees up drop in some oil ,run a 20 to 1 mixture if it starts for a while chech thermostate is not blocked pm me if you need help

Yara said..
I stupidly ran my 5hp Mariner until it stopped after pulling off the fuel hose. Should have stopped it normally. Got away with it for years, but finally it caught up with me, and the piston has seized in storage.

There is a chemical/rust dissolver called "Engine Release" made in Canada, just for this application. Does anyone know where or if it is available in Sydney, or an alternative similar magic potion?

When in storage, is the engine upright on a stand or laying down???

If it is left laying down, ie back of engine on the ground, water and moisture accumulates around piston, cylinder head and water jacket and causes corrosion.

Stored outboards should always be upright on a stand, lashed to an upright or hung up with line.

Aha!!! If the piston was in that position when stored the inlet and exhaust ports would have been uncovered allowing moist air to enter and cause the problem.

With any single cylinder engine (2 or 4 stroke) when stopped it is a good idea to bring it up onto the compression stroke thereby sealing the combustion chamber.

The 4 and 5 hp Mariner/Mercurys are a great little engine. Basicly a Seagull engine with a few updates. The spark plug is in the middle of the cylinder head which allows you to insert a rod directly onto the piston top. If the piston is not right on BDC you could insert a rod (try a timber dowel first) and gently tap on it with a hammer to free the piston. Then work the crank back and forth by hand directly on the magneto gradually freeing it up.

With the engine like it is you will have nothing to lose by squirting plenty of WD40 in there.