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  1. #1
    Sooo...how much for the 780? stimpsonjcat's Avatar
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    The FIRST 96 SLT780 I bought is broked

    I am not sure what went wrong, but I was tuning this ski yesterday and came in to check wash patterns and BOOM, the cylinder I re-honed and replaced the piston in has physical damage as I look in the spark plug hole!

    So I yank it out and pull the head and it looks like the piston caught the edge of one of the ports and broke off a small piece of the piston from the dome to the first ring slot about 3/16" wide and it bounced around and destroyed the head and the piston.

    The ski gave no indication anything was wrong while riding it...so a bit of a shock.

    This ski is in otherwise great condition, so I have it in the garage and intend to yank the engine and go all the way to the crank and back.

    I'll post pics this week.

    If possible I may try to have it running by next weekend, but obviously that depends on what has happened in the crank.

    Has anyone ever seen a piston fail like that before?

    You can see exactly where the piston caught on the port, and the missing piece exactly matches the nick in the port. Is this dumb luck or did I do something wrong?


  2. #2
    MSX 150 guy lives on Mr. GP1800's Avatar
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    Was the port chamfered correctly? Did the ring possible spin on the piston?

  3. #3
    Sooo...how much for the 780? stimpsonjcat's Avatar
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    I used a ball-hone, so it shouldn't have been a sharp edge, but I didn't chamfer all the edges after honing, just ran the ball-hone both directions.

    I haven't gotten the piston out yet, so not sure where the damage is relative to the ring-pin or ring edges.

  4. #4
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    What piston did you install?

    Check to see if the piston ring retaining pin is still in place.

  5. #5
    Sooo...how much for the 780? stimpsonjcat's Avatar
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    IIRC it was a stock (+0) WSM.

    I'll have more info when I get the jug off.

  6. #6
    Sooo...how much for the 780? stimpsonjcat's Avatar
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    So I am remembering now that the front cylinder was running hot when I first put her in the water on the trailer. I discussed this in the build thread, and wrote it off to an orientation issue (steep ramp). Now I am thinking something wasn't right spec-wise, though everything added up according to the manual.

    I used the SBT break-in oil for the whole first tank, but I did not turn off the oil-injection which is synthetic, would this have been a problem?

  7. #7
    MSX 150 guy lives on Mr. GP1800's Avatar
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    Synthetic oil wouldnt cause that

  8. #8
    Sooo...how much for the 780? stimpsonjcat's Avatar
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    STD SBT piston.

    The retaining pins are where they ought to be and the rings were lined up when I pulled it apart.

    It appears the retaining clip lost an ear...
    Click image for larger version. 

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    Here we see what that started...
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    This is impressive damage, never seen that before....
    Click image for larger version. 

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    Here's the damage to the upper cylinder...
    Click image for larger version. 

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    And the lower portion...
    Click image for larger version. 

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    So far I cannot detect any junk in the crank, but I finally had to stop working on it and get a shower and rack time.

  9. #9
    Sooo...how much for the 780? stimpsonjcat's Avatar
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    I did notice some oil in my stator...does this imply a leak in the front seal?

    Click image for larger version. 

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  10. #10
    Click avatar for tech links/info K447's Avatar
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    Quote Originally Posted by stimpsonjcat View Post
    I did notice some oil in my stator...does this imply a leak in the front seal?

    Click image for larger version. 

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    Just that little bit of oil or was there more?

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