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  1. #1
    ripcuda's Avatar
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    When a piston melts a hole...

    where does the molten metal usually go?

    So I've got another project coming. Sun n' Sea Rio StepJet powered by a red domestic Polaris 1200 (1165cc) carb'ed motor. (See Rio info thread here: http://www.greenhulk.net/forums/showthread.php?t=188460 )

    The center piston melted a hole. I'm very thankful the pilot at the time detected the anomaly and shut down the engine quickly. Damage seems to be very minimal.

    The center cylinder and piston have been pulled off the motor. The piston has a small hole in the middle... otherwise looks intact. The cylinder looks fine, no scaring, scrapes or gouges visible. The head is still smooth too. There also is no noticeable damage or debris in the lower crank/rod area either (will know positively upon disassembly).

    So... where did the melted piston go? Does it vaporize or get expelled out the exhaust?

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    I don't have the boat on-hand yet... but I will be pulling the motor and doing a complete rebuild... carbs included (likely cause).

    Cheers!


  2. #2
    Myself's Avatar
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    That happens a little at a time and it gets so hot from being lean the aluminum is actually burned off. If any pieces are large enough to break loose they are expelled out the exhaust system.

  3. #3
    100 MILES OF HEAVEN AND I AM SMACK IN THE MIDDLE KEYZBUM's Avatar
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    Quote Originally Posted by Myself View Post
    That happens a little at a time and it gets so hot from being lean the aluminum is actually burned off.
    I have to disagree...I could be wrong..but... This CAN happen on a new piston in 2 sec's at WOT.
    Just takes a drop of a few psi of fuel pressure to lean out and Blowtorch a hole.
    Polaris are real bad about this BUT they normally take out the MAG cyl.

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
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    Some of the missing piston material certainly was forced through the piston into the crank case. That hole was smaller at first, then got larger until the compression dropped far enough for the combustion heat to diminish. During the time the hole was growing, material would have been going through the hole on each combustion event. At 6000RPM, that would be 100 power strokes per second.

    Think of it as a fine spray of partially melted aluminum particles being blasted down from the piston underside. Where do they go?

  5. #5
    ripcuda's Avatar
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    I'd like to believe that it all goes out the exhaust... but I can picture how the compression above would force the particles downward into the lower crank area.

    So does this commonly cause any damage to be concerned about to the lower case/crank/rod area?

    I do plan to fully rebuild the motor including splitting the cases... and am hoping a new piston/cylinder is all that will need replaced (besides gaskets).

    Cheers!

  6. #6
    Connecticut CrazyA's Avatar
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    Quote Originally Posted by K447 View Post
    Think of it as a fine spray of partially melted aluminum particles being blasted down from the piston underside. Where do they go?
    Up through the transfer ports sometimes and hopefully out the exhaust......and 'round & 'round we go !!!

  7. #7
    I bought a pair of Seadoos a couple of years ago both with melted PTO pistons. Did just the top ends on them, rebuilt carbs and they have both been running trouble free for 2 years now.

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