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Thread: Broken piston

  1. #21
    Senior Member tzrider's Avatar
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    I posted this some time ago but after some more investigation I think the cause
    of this is detonation. My SR500 had some detonation although it was totally standard.
    I think my CDI has gone out of specs or maybe the flywheel/stator is of a bad combination.
    The KEDO CDI manipulator solved the detonation but I will go for a after market CDI.
    Yamaha SR500 48T - Yamaha Tricker XG 250 - Suzuki SV650 MK1

  2. #22
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    Hi tzrider, could you post a photo? I am imagining a cratered out piston crown from pre-ignition , and that does seem unusual in a stock motor. Was the piston the original Yamaha factory one?

  3. #23
    Senior Member tzrider's Avatar
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    Gotsron, please look at page 1 of this topic, pictures enough
    Yamaha SR500 48T - Yamaha Tricker XG 250 - Suzuki SV650 MK1

  4. #24
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    Quote Originally Posted by tzrider View Post
    Gotsron, please look at page 1 of this topic, pictures enough
    I plead incompetence! Oh yes, now I see the previous pages Detonation is absolutely bad for an engine, I have seen warnings for damage to the small end of the rod from the pounding the piston gets among other things. I really can't add much, I don't see any signs of a seizure on the piston, and usually they seize on the skirt first anyway. I've seized a number of pistons without breaking them, and only broke one from excess wear. The vertical line on the cylinder makes you wonder a bit, but possibly rubbed on a broken piece to get that. I'd go with Andy and a casting fluke since everything else looks so good. If your Wiseco goes the same way though I will have to look at the star charts
    Last edited by gotsron; 01-15-2018 at 01:58 AM. Reason: missed page 1 &2

  5. #25
    My first guess was detonation. Second guess was going to be end gap (but the ends of the rings didn't look like the butted up against each other).

    Detonation will absolutely do that to the ring lands.

  6. #26
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    Work in engine reconditioning and that sure looks like detonation to me.Low grade fuel and/or ignition timing

  7. #27
    Senior Member MadMax's Avatar
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    Just for reassurance I'd want to strobe the timing, as you've mentioned, it's possible the stator/rotor/CDI could be a missmatch.

    Max

  8. #28
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    This is for sure detonations.. Pinging or detonations is typically happeneing where you have the least homogen gas mix. (read lean) And that is very often in the circumference in the combustion chamber.
    I once had a highly tune VW beetle engine with 11,5:1 in CR.. it was pinging on fairly high rpms and finally after a year of abuse it started to go a little un-even and it was not possible to adjust idle to an even state.. I took it apart and one of my pistons looked exactly like yours. When pinging occur you have detonations instead of combustion. So what you hear is the flame front from detonations actually exceeding the speed of sound. so yes, you will have much more local stress over this part of the piston where pinging locally occure transferred by the rings down to the segments broken off.
    So what ever reason.. It can be ignition timing, lean mixture or high compression or a combination...

    mcjongs

  9. #29
    Senior Member tzrider's Avatar
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    Just for reassurance I'd want to strobe the timing, as you've mentioned, it's possible the stator/rotor/CDI could be a missmatch.
    Just ordered a puller and will take off the rotor next week. Will let you know.
    Yamaha SR500 48T - Yamaha Tricker XG 250 - Suzuki SV650 MK1

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