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While changing the spark plugs on a new to me 2014 Yamaha outboard I noticed that 3 of the 4 wells have a fair bit of corrosion in them, this corrosion isn't just in the 'outer' surface of the well but extends to the machined mating surface against which the spark plug metal washer would seat.

Is this a concern? I'm guessing the uneven corroded surface might mean that the spark plug doesn't seal/seat well causing some leakage and reducing the engine compression?

Any advice on how to clean? I don't want to go to down with a drill-mounted wire brush seeing as brush metal bristles might end up in the combustion chamber. I tried to scrape it a bit with a pick but also worried I might mar up the surface and do further damage

I'm guessing this is a common issue, any thoughts or advice?

Some pics for reference:

Two of the corroded wells: enter image description here enter image description here

Upper spark plug well is nice and clean (how it should look like for reference): enter image description here

wild.coast
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You could cut the threaded part off an old spark plug, knock out the ceramic, roughen up inside the hole, and fill the hole where the ceramic was with something like chemical metal. This can be screwed into the plug hole. Cut a slot in one end for a screwdriver, making sure you make a slot in the metal part.

Plug the hole with this, then you could use a rotary wire brush to clean the hole. Only use the wire brush in reverse, not in a clockwise direction, so that it would tend to unscrew this plug rather than screw it into the cylinder. Keep checking that the plug isn’t unscrewing.

Once the plug hole is clean, unscrew the plug and lift out. You may need a magnet to lift it out if you can’t reach it easily. Like I said, cut the screw slot through the metal of the plug, just in case the chemical metal slot gets worn away by the wire brush.

HandyHowie
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