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  1. #1
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    Default Piston damage - PICTURES

    Just got the top end off and found some decent damage to the mag piston. This cylinder showed 20psi. The PTO showed 125, and a lot less damage. A little background - I bought this ski (99xpl) about a month ago. It still had the grey fuel lines and some running issues. Replaced entire fuel system, rebuilt the carbs, and rebuilt the jet pump. The ski ran awesome for about a tank and a half of gas. While riding last week I lost power, and limped it home. A compression test then brought me to this point. I'm sending the jugs to full bore tomorrow. I just wanted to get some opinions on what you think caused the failure. The previous owner told me this was a new top end with around 30 hours on it. Although its clear the top end has been apart before, (rtv silicone lathered on exhaust components) I don't know how accurate the hours are. My guess is that the previous owner had been running it lean with the grey lines and plugged up carbs that ultimately led to this. Even though I recently addressed the issue, the damage was likely already done. I checked the plugs after every ride and they were always a nice brown color. But the piston wash seems to suggest a lean condition. What do you guys think?

    edit to add - The cylinders have very little scoring on them, they have a good amount of cross hatch still visible.

    damage 2.jpgpto.jpgdamage 1.jpgwash 1.jpgscore.jpg
    Last edited by JonRusso89; 08-01-11 at 09:36 PM.

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  3. #2
    Dr Honda's Avatar
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    The damaged doesn't look like a typical lean run. Dont' get me wrong... it was lean... but the "Cokeing" on the rings, and skirts looks like the wrong oil too.

    What oil was he using?

    Also, since there is "Mechanical" damage to the piston... what does the RAVE look like? If the PO did a top end... I wonder if they shaved the RAVE. If they didn't... it may have touched the piston.
    Tony
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    I'm not going to be able to do vinyl or carbs for a while. Starting my new job is taking a lot of time

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  4. #3
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    He told me he had used only XPS. It was pretty low when I got it, and one of the first things I did was drain the tank, bleed it, check all the lines, and check the adjustment. I filled it with XPS full synthetic. But only ran about a quarter of the oil tank before this happened.

    The rave was also a concern of mine, it has a couple small nicks in it, but it looks more like it had been dropped at some point than like it clipped the piston. Also If I feel inside the exhaust port with my fingers, it feels like there is more than enough clearance. I think I'll shave the raves regardless when I do the rebuild. What else do you think I should look out for after the rebuild? I would like to do everything I can to ensure longevity.

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    Well... if I was doing the top end... I would do a leak down to make sure that there aren't any case leaks. I would also check the reed valves for damage. (could cause it to run lean)

    Oh... One more thing... after you get it back together... verify the squash band. If it's too tight... it will cook the piston edge.

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    I can see "Boiling" on the exhaust edge... but there is more fracture damage than melt damage. I wonder if the ring rolled around, and grabbed the port. Is there a ring gap on the top ring, around on the other side?

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  7. #6
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    Quote Originally Posted by Dr Honda View Post
    I can see "Boiling" on the exhaust edge... but there is more fracture damage than melt damage. I wonder if the ring rolled around, and grabbed the port. Is there a ring gap on the top ring, around on the other side?
    I think that is a possibility. I just pulled the pistons, don't know how I didn't notice this earlier but there is nothing at all left of the top ring.

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    Quote Originally Posted by JonRusso89 View Post
    I think that is a possibility. I just pulled the pistons, don't know how I didn't notice this earlier but there is nothing at all left of the top ring.
    Sorry... I thought I saw what was left of the ring, in the top ringland.

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  9. #8
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    Looks more like you lost the locating pin. The wash on the other piston don't look to bad but the type of oil and or pump setting should be looked into.

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    Quote Originally Posted by 470 View Post
    Looks more like you lost the locating pin. The wash on the other piston don't look to bad but the type of oil and or pump setting should be looked into.
    Thanks.. The more I look at it, it does appear to me that the top ring rotated and caught the exhaust port. The upper ring groove is demolished, and there are marks (hard to see in the pic) that look as if the ring got slammed around in the groove before it broke apart. As for the oil pump, it was adjusted along with the carbs throttle cable after I rebuild the carbs. I ran XPS full synth, and I'm pretty positive the PO ran XPS as well, I just don't know whether it was synthetic or mineral.

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  11. #10
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    If there is any aluminum from the pistons that got transferred to the cylinder walls you can get it off real easy with muriatic acid and a cue tip. Just take your time. Make sure there is still a chamfer on the ports or you will snag a ring. Looks like you may have gotten away with an easy top end job as long as those cylinder walls clean up nice.

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  12. #11
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    Quote Originally Posted by 470 View Post
    If there is any aluminum from the pistons that got transferred to the cylinder walls you can get it off real easy with muriatic acid and a cue tip. Just take your time. Make sure there is still a chamfer on the ports or you will snag a ring. Looks like you may have gotten away with an easy top end job as long as those cylinder walls clean up nice.
    Considering the failure I think I did get off pretty easy, I was shocked at how little damage the cylinder sustained. Just some very minor scoring. I sent them out to full bore today anyway. But thanks for the tips.

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  13. #12
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    As a future reference. If the the scoring or damage is below the transfer ports. You can just run it will no ill affects. I've run a few rotax motors with dime size chunks missing under the ports and never had a problem with them. One has almost 20,000 miles on it and it spends lot of time north of 8300 rpm.

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