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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

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    I was going to suggest a metal rotary brush, but you bring up a VERY GOOD point about getting bristles down into the bore. I think your best bet is to use a scotchbrite pad to get the outside and use a thread cleaning die to clean the threads. This is just my thinking, though ... someone else might have a much better idea. Commented Nov 5, 2022 at 23:01
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    As much as I think you'd like to avoid doing it, removing the cylinder head is probably your best bet here. You do want to get that rust removed but you don't want to get it into the cylinder.
    – jwh20
    Commented Nov 6, 2022 at 1:08
  • Id use compressed air (through the intake) while cleaning the well.
    – Martin
    Commented Nov 6, 2022 at 12:54

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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.

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  • Brilliant advice. I ended up wrapping a piece of paper towel with a thin metal wire and shoving it in (the wire allowed me to pull the paper towel back out). This worked well enough but your advice is so much better because it's a tighter seal and you don't have that wire getting in the way of the rotary tool/brush.
    – wild.coast
    Commented Nov 6, 2022 at 8:57
  • I guess another option is to just get a M12x1.25 bolt and cut off the heat or just buy some M12x1.25 threaded rod and cut to length. Then grind a notch to be able to screw/unscrew with a flat head screwdriver. A bit more reliable having solid metal throughout the 'plug' instead of using a cut-off spark plug with putty metal - which as you say could be a bit soft... Another trick I realize is to turn the flywheel to set the piston at TDC so there is no risk of the threaded plug from falling into the combustion chamber!
    – wild.coast
    Commented Nov 7, 2022 at 22:54
  • @wildcoast Are these spark plugs definitely M12? I know M14 is a common thread size for spark plugs. I didn't want to suggest a specific size of thread in case damage was caused to the head. That is why I thought using an old plug was a good idea.
    – HandyHowie
    Commented Nov 8, 2022 at 9:22
  • Definitely M12x1.25, measured and chased the threads with a thread chaser to clean em using that size and pitch. I know in automotive M14 is common, but this is an outboard so I'm guessing the smaller engine blocks have different standard sizes. 1.25 for M12 is also a super fine pitch which was curious.
    – wild.coast
    Commented Nov 9, 2022 at 23:48

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