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  1. #1

    98 gp800 top end destruction

    When I bought the ski 3 years ago the previous owner told me that it dropped a power valve and he had to rebuild the top end. He told me he had the cylinder re-plated and rebuilt by a shop. I was having problems with starting so I pulled the carbs. The carbs looked good so I checked the compression and to my surprise the mag cylinder was pretty much zero. I pulled the head to find a chunck out of the piston near the power valve. Then I realized the huge piston clearance. Spec says .003-.004 inches I measured .024 inches. I dont know how it stayed together this long. My question is what is the cheapest reliable way to rebuild. Should I replate?....sleeve? It sounds like it was overbored .5mm and they threw in stock pistons. If it is overbored do I need to change jetting? It is all stock and that is how I would like it to stay. The only mod is oil pump removal. I have the modified power valve links in it now so the valves did not drop. Also the head got dinged up a little. Can I clean it up or do I need a new head?


  2. #2
    Yamaha artisan Cutlass's Avatar
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    You most likely can clean up the head. Just smooth out the dome with sand paper. It doesn't have to be perfect, just no sharp edges. Stay inside the dome though. don't let the sand paper sand away at the sealing surface of the head.
    The cheapest top end is probably SBT. I think those cylinders aren't plated though. Most people would recommend sending the cylinder(s) to Millenium Technologies, and have them replate and size the cylinders and pistons. Just send them the cylinders and they'll sell you the correct size piston.

  3. #3
    Will I need to re-jet for the oversize?

  4. #4

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    Can't speak specifically to a gp800, but for outboards or dirtbikes, no...not necessary. If anything, maybe a tiny adjustment of the screws, but likely not much of anything.

  5. #5
    Yamaha artisan Cutlass's Avatar
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    ^^^exactly. no rejet, maybe small screw adjustments.

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