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  1. #1
    TX21's Avatar
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    cylinder #2 loose spark plug lead to blown #1 cylinder?

    Let my friend borrow my ski and the motor took a crap while he had it. I got it back and the #1 cylinder was toast. Rings were intact wrist pin was good mag bearing ...good, crank bearing was good. Only things I can see were the #2 spark plug was loose #2 #3 cylinders were perfect. #1 had the piston skirt blown apart. Carbs looked good. Not sure why the piston skirt ate it. I built the motor about 5 years ago and its been solid since. Prob have 40 hours on it.


  2. #2
    Prism's Avatar
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    What brand pistons were used in the last rebuild? Just curious...

  3. #3
    TX21's Avatar
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    Wsm platinum and I think it was 2009 or 2010 when I built it. Kinda thinking it may be the wsm pistons since I have heard of this before.

  4. #4

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    oil problem? fuel delivery problem? Both cause a lack of lubrication, and a possible piston failure.

  5. #5
    TX21's Avatar
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    Pre-mix, supposed to be correct not sure how to verify that. Carbs look fine so I guess if he ran it out of fuel it could have sucked air in on the #1?

  6. #6

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    Pressure test the engine, front seal might be bad?

  7. #7
    TX21's Avatar
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    Crap I guess I could have done that. Too late I already took it apart. Seals looked good but that's not the proper way to check them, I've got new ones that are not good too. So what I gather is the #2 plug being loose could not be related to a #1 problem right?

  8. #8
    Myself's Avatar
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    Skirt broken off is usually too much clearance. The piston rocks and slaps in the bore until it finally gives.

  9. #9
    TX21's Avatar
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    Makes sense I guess. Had prob 40 hours on it but he may have pushed it further than I do.

  10. #10
    Prism's Avatar
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    Do you have or can you take a picture of the broken piston? Does the piston crown look melted or like there was a fuel issue or is most of the damage concentrated around the broken skirt area? The loose plug will not cause damage to another cyl IMO. Any traces of piston wash to read? A picture is worth a thousand words. The bore was a standard bore correct? Even if you put the smallest OEM piston in the biggest cylinder, there should not be too much clearence to cause a failure. Alot of the race guys use the smallest OEM pistons on all their cylinders to keep them from sticking during a race. Pistons usually don't make noise or break at 7000 rpm until the clearence gets crazy. Like over .008" or almost double the average spec for even a forged piston. I'm not sure if WSM comes in different sizes or not... and I don't think they're forged. Maybe just a flat out piston failure? Post a pic if you get a chance.
    Last edited by Prism; 07-28-2015 at 06:27 PM.

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