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  1. #1
    Sly Fox DKFox's Avatar
    Join Date
    Feb 2009
    Huntsville, Alabama

    Need some piston analysis after engine overheat. Please see pictures.

    1998 SPX with 787 engine.

    Fresh top end rebuild.

    Finally got it in the water. It was bogging down. I was concentrating on that and didn't watch my water flow close enough. Under the seat I didn't notice that the clear tubing I had used as cooling lines apparently couldn't stand the heat. A couple of them melted and the engine shut off after 3 minutes or so of idling. (There is no dash alarm or buzzer since this was a former race ski.)

    Got it home and checked compression. They were both 170 psi right after the rebuild. Now the Mag side is at 145 psi and the PTO side is at 30 psi! Tore the engine on apart and was expecting to see scorched pistons or at least broken rings. Am seeing neither! (See pictures.) No scratches or burn marks at all in the sleeves. All bolts were tight and gaskets new. (Was watching for lose bolts as I was uninstalling.) Did the heat just cause the PTO rings to lose their ability to expand? Is that why I lost compression?? Thinking about trying ring replacement only. Already at 83.50mm and sure don't want to sink $400 into another top end. This one didn't live long enough!

    Comments are appreciated.

    P.S. Yes... I'll use heavier and thicker cooling line this next time. Seeing the flow is a plus whenever it is on the hose. But doesn't help at all when riding and the seat is on!
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  2. #2
    are those rfi pistons? or aftermarket pistons for an rfi? just asking because of the port in the side of the piston. The person who did the machining are they mostly a 2 stroke engine machine shop or an automotive shop that mostly does 4 stroke engines? asking because of the way that they are bored is different between 2 and 4 stroke engines. also depends on the piston to wall clearance.
    doesn't look like an overheat or a lean condition. what does the cylinders look like? is there any metal transfer is the cross hatch still there? what are the other variables, like carbs, jetting, needle settings, what head ( compression), base gasket thickness.
    some more info hopefully can figure it out.

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