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  1. #1

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    WTF happened to this engine?

    I had started another thread trying to locate the source of the problem but now it is apparent it is my engine. Unfortunately in true amateur fashion I forgot to take more pictures while I had access to the ski. I will get some of the piston/cyl later.

    The top of the PTO or MAG (dunno which is which) piston was broken up around the perimeter with some pitting across the surface. The top ring was roughly 90% intact although broken in multiple places due to being compressed by whatever was impacting the surface of the piston. Hopefully the picture of the head can help you picture this. The cyl wall seemed almost perfect aside from one very slight score.

    So far from the research I have done I can conclude one of two things. Something F'd the crankshaft which has forced the piston to contact the head. Or some fuel condition caused that cylinder to over heat causing that cylinder to fail somehow.

    I read something about possibly indexing the crank? Would this allow me to tell if the crank is in good enough condition to not worry about a bottom end rebuild? If not is there any way to test the bottom end without tearing it apart? I am really hoping this is just a fuel related issue and I can get away with a top end rebuild. I will post more pics later this week, but would really appreciate some input on where to go from here. Currently we pulled the head and now the thing sits in a friends garage.
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  2. #2
    All those "cuts" in your Head's Squish Band and probably similar ones on the Piston Top are from a Piston Ring which ended it's life too early and broke up. Probably from overheating, Ring Pin loss/Rotating Ring, detonation, etc.

    You have to ask yourself, 'Where did all the metal frags go' ?
    Out the Exhaust or possible down in the lower end, lodged between the seals and bearings.
    You could take a chance and just do the Top End, or clean out the bottom and avoid possibly throwing a Rod through the Cases if the Lower End finds a chunk of Piston Ring stuck in a Bearing. Cases with Rods sticking out aren't easily repaired....

  3. #3
    SURFnTURF's Avatar
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    Yes unfortunately that requires a full tear down and inspection at the very least. Without seeing the piston I would say detonation is not the culprit or overheating as no visible signs are present. The spark plug electrode is it still intact ??? as this can be the cause of what I see.

    I would say your rings have let go for some reason due to fatigue or improper installation which caused it to be lodged between the piston and head as it travels through the compression stroke. What are the bores like especially on the exhaust side ??? Are the pistons worn away on the edges or just chipped and broke into pieces ???? Really need some more pictures to be able to diagnose it better.

  4. #4
    Richieb's Avatar
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    Sometimes a flappy piston in the bore will jolt the rings about enough to shatter them too.

  5. #5
    "Tearing it up on the water" seadoobutch787's Avatar
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    Well mine looked like that but it was due to deto. I have a stock 787 head i will sell you if your interested.

  6. #6

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    Thanks for all the replies guys. A couple things I forgot to mention..

    This is the 787 engine from a 97XP. The other cylinder appeared to be in good condition, compression checked out at ~145 (the cyl closest to the front of ski), the one that blew had 0. The temp sensor that threads into the head broke where the cable meets the sensor. The original fuel lines are still in place, and to my knowledge the filter on the carbs has never been checked.

    seadoobutch, if I do rebuild this thing I will be very interested in that head. I will let you know in the next week.

    I will update this thread with pictures, hopefully you guys will check back then.

  7. #7

    Wink

    Quote Originally Posted by located_in_MI View Post
    Thanks for all the replies guys. A couple things I forgot to mention..

    This is the 787 engine from a 97XP. The other cylinder appeared to be in good condition, compression checked out at ~145 (the cyl closest to the front of ski), the one that blew had 0. The temp sensor that threads into the head broke where the cable meets the sensor. The original fuel lines are still in place, and to my knowledge the filter on the carbs has never been checked.

    seadoobutch, if I do rebuild this thing I will be very interested in that head. I will let you know in the next week.

    I will update this thread with pictures, hopefully you guys will check back then.
    FYI,

    The cylinder near the Magneto (towards the front of the ski) is referred to as the mag cylinder. The other one next to the pto shaft is referred to as the pto cylinder.

  8. #8
    AKA "The Guppy" JeffTurbo601's Avatar
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    looks like the needle bearings went...they go through the port and wind up on the top of the piston inside the dome

  9. #9
    96XPSS's Avatar
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    Quote Originally Posted by JeffTurbo601 View Post
    looks like the needle bearings went...they go through the port and wind up on the top of the piston inside the dome
    +1

  10. #10
    SURFnTURF's Avatar
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    If it was needle bearings you would have long straight indents on the head and piston. I cannot see 1 of those in the picture.These are typical of broken ring fragments and small pieces of cast piston being hammered around in the combustion chamber. Either piston or ring take a pick.

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