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  1. #1

    piston hole - what causes this?

    I was out on my gp1200r for the first run of the season last saturday. After 2 hours of smooth riding it started bogging down all of a sudden and would shake violently, sounding like crap at anything above idle. Compression on bow cylinder was 0psi, middle cylinder 100psi, stern cylinder 110psi. Took off the head and found this hole through the piston. The cylinder wall is scored near the exhaust port as well. I topped off my tank when winterizing last year and added sta-bil fuel stabilizer as i've seen recommended. I was burning through that tank of last years gas when this happened... any ideas what would cause this?

    Everything seems fine in the crank case, my plan is to buy the shopSBT cylinder exchange and replace all pistons and cylinders...
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  2. #2

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    First guess would be your oil line to that cylinder popped off.
    That or maybe a ring hung up on the exhaust port.
    The guys that really know will chime in shortly and either give me a +1 or laugh at me.

  3. #3
    mudslanger's Avatar
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    I dont believe i would replace all cylinders and pistons through SBT just the bad one and if it has a hole in it they want exchange with you they will charge you extra for core cylinder. check on here for a used cylinder or core cylinder. as long as the other cylinders are not messed up with scratches they should be OK.

    AS bad as that cylinder looks you should probably do a total tear down and check the crank bearings.

  4. #4

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    I thought SBT sleeved their cylinders.

    My thoughts are to buy a new piston to match that cylinder, and send them both to Mellienium to have the cylinder reconditioned and the piston for them to match it. It is probably cheaper, and you have a much better product.

    The deal is why did it happen in the first place. It appears that the ring did catch the exhaust port, but what caused that to happen?

    Is is a stock boat? Was your gas (octane) lame from last year? I cannot tell from the picture, but in the middle of that piston crown, has the metal been eaten away like there is a small crater there?

    Because the piston has been torn to pieces, it is reasonable to believe that parts of it may have fallen into the case. A total tear down would not be a bad idea.

    Out of curiousity, did you dump any of your water from the head over the side rather than have it go through the exhaust pipe.

  5. #5
    "First guess would be your oil line to that cylinder popped off."

    --- This ski was converted to premix before I bought it, I mix 40:1.


    "I dont believe I would replace all cylinders and pistons through SBT just the bad one and if it has a hole in it they want exchange with you they will charge you extra for core cylinder. check on here for a used cylinder or core cylinder. as long as the other cylinders are not messed up with scratches they should be OK. "

    --- I've heard suggestions to only do the faulty cylinder, and suggestions to replace all of them. I was going to do all of them because the majority of the replacement work is in the teardown of the engine (might as well do them all while I have it apart). Also, not sure about the SBT exchange. Additional advice on this subject is appreciated.


    "Is is a stock boat? Was your gas (octane) lame from last year? I cannot tell from the picture, but in the middle of that piston crown, has the metal been eaten away like there is a small crater there?"

    --- Not stock, the owner before me added the jetworks valve, changed it over to premix, added performance air filters, and removed the catalytic convertor (installed.. d-plate?) . It was a full tank from last year, premixed at the end of the season with the recommended "winterizing" dose of Sta-bil marine added to it. I'll have to go feel the top of that cylinder... but it does look like theres an indentation near the spark plug location...


    "Because the piston has been torn to pieces, it is reasonable to believe that parts of it may have fallen into the case. A total tear down would not be a bad idea."

    --- If I'm not very experianced with engine work, wouldnt this be much more difficult and require me to mess with engine timing? Is it practical that the piston head fragments would have made it into the crank case and not all blown out the exhaust? Should I check exhaust components for the pieces or would they probably blow completely out of the ski?


    "Out of curiousity, did you dump any of your water from the head over the side rather than have it go through the exhaust pipe."

    --- Nope, never diverted any of the cooling system


    --- Additional thoughts: Could this be a heat issue? I went to check my water intake strainer last night and one of the screws is inaccessible because of the interferance with the ride plate, guess I'll have to wait to check that....

  6. #6
    PolarisNut's Avatar
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    What do the other pistons look like?

  7. #7
    The other two piston heads look unscathed and their cylinder walls feel smooth. Heres a picture of the stern piston, I can get more later if needed.
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  8. #8
    Happily Self-Employed WFO's Avatar
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    last one I had like that was caused from water leaking past the head gasket

  9. #9
    Steveard's Avatar
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    "--- If I'm not very experianced with engine work, wouldnt this be much more difficult and require me to mess with engine timing? Is it practical that the piston head fragments would have made it into the crank case and not all blown out the exhaust? Should I check exhaust components for the pieces or would they probably blow completely out of the ski?"


    My 1200 failed close to the same way. I did a full teardown and found a lot of metal in the lower crank case.

  10. #10
    Matt Hill's Avatar
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    I vote spark knock.

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