SBT Jetski Engines, Parts & Accessories

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  1. #1
    Sep 2004
    Georgetown, TX

    Default Why only 1 cylinder bad?

    I bought a broken 96 Yamaha Waveraider. It had no compression on the front cylinder. I recently pulled the engine, and then pulled the head off the engine. I've attached some pictures of the cylinder and piston. Pic 1 is a picture of the piston taken through the exhaust port. It's obvious there's been a lot of scraping going on. :( Pic 2 is a picture of the top of the bad piston. You can see where the edge of the piston has been rolled over. Pic 3 is a picture of the cylinder. Aluminum has been deposited all over the cylinder wall, but it is especially thick at the top. It seems fairly obvious to me that the piston overheated and expanded enough to start scraping against the cylinder walls.

    The questions I have are:

    1) The person I bought the ski from never mentioned it having seized. I know her well enough that I doubt she would have intentionally misled me. Is it possible for a piston to scrape that badly without ever seizing?

    2) The Waveraider is a single carb engine with oil injection, so I would think that both cylinders would be getting the same gas/oil mix. How could one cylinder overheat that much and not the other? The other cylinder looks fine. The piston shows no signs of scraping, the cylinder wall looks unscratched, and I was getting good compression on it before I pulled the engine.

    3) Is it possible the crank has gotten twisted? If so, how can I check it?

    4) Does the cylinder have to be bored, or would it be possible to just hone it to remove the accumulated aluminum? If it has to be bored, how much should I bore it?

    Thanks guys.


    [Additional info 7-8-2005] I removed the cylinders and inspected the bad piston some more. Now that I can see the entire piston, I'm thinking less that the cylinder was running hot from a bad fuel mix since I don't see any evidence of heat damage to the piston, and more that it somehow was not getting lubed properly, maybe because it was injesting some water from somewhere. See pic 4.

    This brings up a few other questions:

    5) Where was the water coming from? The inside of the engine compartment was bone dry when I bought the ski. My first thought was a leaking head gasket letting cooling water into the cylinder, but I can't see any obvious evidence of this. My next thought was that the cylinders casting itself was bad somehow and cooling water was getting into the cylinder through a hole or crack in the cooling passages of the cylinder that was letting water leak into the crankcase. Does anyone have any knowledge of this ever happening?
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	1) bad piston side thru exhaust port 1.jpg 
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Name:	2) bad piston edge.jpg 
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    Last edited by wmsky; 07-08-2005 at 04:47 PM. Reason: Additional info

  2. #2
    Feb 2005
    Adamsville, AL

    Default Re: Why only 1 cylinder bad?

    Water can only get into the cylinder thru one of three ways... the intake, the exhaust, the cooling system... from the intake if swamped, from exhaust if water box is busted, from cooling system if connection is loose or hose has a hole in it. Otherwise, water is coming straight thru the hull (drain plug in?) As for hole in the piston and scapes....looks like this thing was run without oil. If you can hone the scratches out of the side you might be lucky to clean up the head and just put in a new piston.... GOOD LUCK! Be sure to check side play of connecting rods...could have bad rod bearings too.

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