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I own a quite old motorcycle - Aprilia 125 Pegaso and recently it stopped working, I figured out it could be the spark plug so I cleaned it and it started throwing sparks again but stopped working once again. This time I decided to bend the finger a bit out so I could actually clean it well and bent it back, but can this damage the engine? I know the timing is really important mainly in a high rpm engine so I'd rather ask first.

TLDR: Will slightly bent spark plug finger break an high rpm engine?

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  • Welcome to Motor Vehicle Maintenance & Repair! Nov 26 '21 at 13:14
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    If you bend it enough to make it metal fatigue and break off inside the engine, then oh, yes.... Nov 27 '21 at 21:33
  • Stop wasting your time! If the shoulders on the spark plug tip aren't square, it's new plug time. And you probably gapped it wrongly anyhow.
    – Tim
    Nov 28 '21 at 14:58
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I am yet to see an engine this tight that the piston can hit the spark plug ground electrode even if it is bent straight.

Even if they somehow meet, the damage will be minor and not worth the worries - unless the electrode breaks and falls off in the engine.

On the other hand, cleaning the spark plug doesn't really require bending the electrode.

What you probably failed to get right is the spark gap. The proper engine operation really depends on it and the wrong gap can promote other problems (like, e.g. the coil/capacitor/CDI burning).

The proper spark gap for the majority of engines is 0.6mm .. 1.3mm and a 2-stroke motorcycle probably expects a gap in the lower end (0.6 .. 0.7mm).


What you can do?

  1. Replace the spark plug with a brand new one. It is not too expensive.
  2. Run the bike for a while (30-50km)
  3. Get the spark plug out and compare it to a "spark plug color chart" (search these words for pictures). See if the carb or the ignition system needs fixing.
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  • Thanks for the detailed explanation Nov 26 '21 at 21:38
  • Any higher compression engine (like motorcycle or high performance engines) run the risk of the piston coming into the contact with the ground strap if it is pointed straight. Nov 27 '21 at 2:15
  • Right, fiddling over a spark plug is like trying to repair a common household power receptacle. They are 60 cents ($3 for the really good ones). Nov 27 '21 at 21:34
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Perhaps. After bending the spark plug electrode you really should replace the plug. These things are not designed to be bent more than a tiny amount to adjust the spark gap. By bending it enough to somehow clean under it you have almost certainly weakened it and made it prone to breaking off. Having a bit of metal in your cylinder will not be good for the engine.

Get a new replacement spark plug, set its gap to the specified size using a proper gapping tool, replace the plug in the engine.

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Buy a new plug. Cleaning does not fix the problem of spark "leakage" up in the annular space. I used a test instrument that permitted looking into the plug annular space using a mirror in the pressure cabinet; You could spark the plug while under (air) pressure. As air pressure increased ,the spark no longer occurred in the gap but up in the body. For a new plug this pressure was higher than a plug that had been cleaned (it was long ago and I do not recall the air pressures). I learned this too late; I sold a '49 Olds standard transmission , it was fairly fast . The guy I sold it to put in new plugs and beat '57 Chevys ( Yes, they are old but the chevy was about the fastest factory car at the time).

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  • That! Spark plugs are cheap - much cheaper than a new engine
    – Aleks G
    Nov 26 '21 at 19:56
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Possibly yes, depending on the engine and how far the ground electrode is bent out. If the electrode is bent out too far it could come into contact with the piston and break off into the engine, which would cause significant damage. repeated bending back and forth can also weaken the electrode and cause it to break, so if you've done a lot of bending back and forth I'd suggest you replace the plug to be on the safe side.

Next time you want to clean a plug use a brush and don't bend the electrode. Use a spark plug cleaner, then use a spark plug gap tool to adjust the gap.

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    Have spark plugs changed that much? When I used to ride motorcycles I had a tool made by Champion, a famous spark plug maker, It had feeler gauge blades, a file blade, and a tool for setting the gap by bending the ground electrode. Of course you had to use common sense and not break it off or replace it in the engine with a crazy big gap or in a deformed state so it would hit the piston. Nov 26 '21 at 20:28
  • Is there spark whether the spark plug was replaced or not? If spark still occurs then there may be a fuel problem.
    – F Dryer
    Nov 29 '21 at 20:24

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