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The story is that the Mrs has got a little 2T scooter and managed to tank it up with some naff petrol that probably had ethanol additive or something and was sitting around in the garage for a few months. So after driving about as far as the end of the driveway it conks out and wont start.

So we drain out the petrol, refill it with good, try everything to start it, including manually choking the airfilter etc., but no joy.

I think, ok it's the sparkplug it's probably wet, so I go to try and remove it. There was not a hope of getting that thing out. I tried everything but it was well and truly rammed in there, and no tools I had got me enough torque to remove it.

So the service guys who had the bike in last time show up, after the Mrs order them to pick the bike up, and when we said 'we can't get the sparkplug out', their response was 'nah there's no way that's coming out manually' so I guess they put it in last time with a pneumatic wrench.

Is this normal? Isn't this like some kind of fault on their part? I mean, I would have thought that those scooter sparkplugs should be inserted so that you can service them on the road even?

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    I'd try to heat it (perhaps an electric heat gun, no oxy-flame or other harsh measures) without damaging the surroundings. Perhaps use some cover to protect delicate materials (cables, plastic..), a fiberglass blanket would be the optimum. The engine head is most likely aluminum, which has a higher thermal expansion coefficient than the steel spark plug thread.
    – Martin
    Commented Apr 22, 2020 at 10:03
  • Thank Martin, well those guys took the bike away and to do some other mods/repairs anyway, but i am a little suspicious they literally aren't tightening things up to the point of being unserviceable deliberately so the Mrs has to take it off to them
    – Frank
    Commented Apr 22, 2020 at 10:28
  • On a separate note from my answer below, your issue is a fueling issue, not a spark plug issue, as far as how the bike is running. It doesn't mean it wasn't time to change out the spark plug, but it is definitely time to get the carb cleaned and reconditioned.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Apr 22, 2020 at 11:06
  • Heat it up with a propane torch if you can, then try again. If no joy heat cycle it, heat up, let cool, do this several times then heat again and try removal.
    – Moab
    Commented Apr 22, 2020 at 15:20

1 Answer 1

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It probably is that the installer of the spark plug did over tightened it, but it doesn't mean they used an impact gun. There may be other things going on here which has seized the plug in the hole or is making it very hard to turn.

First of all, there are recommendations for plug tightening. The following chart from this website shows:

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The same site states:

Tapered-seat spark plugs should be installed by hand until finger-tight. Then use an additional 1/16th turn applied with a spark plug wrench to ensure the spark plug has been tightened to the manufacturer’s recommendation for a 14mm thread. Alternatively, using a torque wrench, tighten the spark plug to approximately 7 to 15 lb. ft. for 14mm threads and 15 to 20 lb. ft. for 18mm threads to ensure a gastight seal. Overtightening a spark plug will stretch the plug body and potentially result in breakage upon removal.

For gasket-type plugs, install the plug until finger-tight. Then if the plug is being installed with a new gasket, use a spark plug wrench to apply a 3/8 to 5/8 turn – depending on head material. (See chart below.) When reinstalling a used spark plug with a used gasket, only ¼ turn is necessary, regardless of head material.

With that said, it's easy to see how a spark plug could get over tightened if the installer isn't paying attention to what they are doing. Also, if the installer didn't ensure the threads were clean of carbon, this might exacerbate the situation. Would over tightening be the only thing goin on here? I don't believe so.

What is really going on here is probably two fold.

First, the plug being over tightened, for sure. There is the possibility the installer has tightened it tight enough to deform the threads of either the spark plug or the hole, making it much harder to remove.

Secondly, depending on how long it's been since the plug was changed in the first place, there may be a bit of "space welding" going on. This would be where two dissimilar metals have some galvanic corrosion may have taken place, which has caused the threads of the hole and the spark plug to seize together, making it near impossible to remove. This is why I always give a strong recommendation to use anti-seize on plug threads to help prevent just this type of issue. anti-seize also helps to seal the threads and to increase electrical continuity between the plug and the head, so is a win-win all the way around.

The situation with your bike is ugly, no doubt. I don't believe any mechanic in their right minds would use an impact to tighten a spark plug, if they could even get the spark plug to survive such an event. It is very easy to over tighten them, though. A 3/8" drive ratchet provides more than enough torque to get the job done, yet unwitting mechanics will use 1/2" breaker bars and man handle things into place. Very ugly.

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    Thanks Paulster2. Mrs WhatsApped me earlier saying the mechanic did find the problem as being the plug, by the way, so I messaged her back telling her to tell them to not overtighten. The plug was new and only in the bike since last autumn. Agreed they should clean carbs. Or if she likes 2t maybe I just buy her an nsr500
    – Frank
    Commented Apr 22, 2020 at 11:45

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