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  1. #1
    Yamaha artisan Cutlass's Avatar
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    Honing question GP1200 with PICS

    So I tore my 65U GP1200 engine down to replace the rear crankshaft seals and found my #3 piston badly scored up(must have been very lean from the blown out rear crank seals). So I thinking about replacing that piston (maybe all 3), honing the cylinder(s), and replacing the rings on all 3 cylinders.
    Question is, do I need to do a really light hone or an aggressive hone in order to get the new rings to seat properly??

    Also what is the stock/standard bore size on the 65U? I measured the piston as best I could with my vernier caliper and looks to be about 84.02mm.

    Here's some pics of the carnage.


    1200cc yamaha 2 stroke


    the scored piston


    Ok piston


    cylinder needs honing


  2. #2
    20 feet Boy's Avatar
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    If you are in the motor I would replace all of the piston and bore it 5mm over. Its not worth it to do a haft ass job enless your selling it. Most of the time it will come back to bite you. Just my thought. All so how many hr on the motor. If it has more then 150 hr to 200 hr I would go thew the hole thing. I dont know that much about tripples, But when they blow it not cheap.

  3. #3
    Yamaha artisan Cutlass's Avatar
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    Thanks for the response! Yeah it has 163 hours on it, so a full rebuild is probably a good idea. I've been researching and lurking on here a bunch lately. It looks like this thing is still at the standard 84mm bore on all 3 cylinders.

    I'm definitely thinking about half assing this thing back together. Maybe I need to learn the hard way.

    My plan so far is to try to find a used OEM piston(or get a new OEM piston), hone just that cylinder, new rings on all 3 pistons, reassemble the engine with oem gaskets/seals+3bond 1211, new plugs, Jetworks mod, rebuild the carbs, and install a primer kit.

    But I'm definitely looking for suggestions and opinions...

  4. #4
    Flying Scotsman's Avatar
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    It looks like the bad bore has light aluminum deposits that can be removed and the cylinder honed.If the bore is scored and you need to bore it then do all 3you are better of with 3 pistons the same size.I would hone the bad sleeve and measure all 3 bores and pistons to make sure clearance is in tollerance and then decide what to do.Measure the pistons then decide if they need replacing.New rings on all 3 for sure.

  5. #5
    Yamaha artisan Cutlass's Avatar
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    Yeah that makes sense. I'm gonna try to find some time this afternoon after football to head out to the garage and work on it a bit. I let you guys know how it turns out.

  6. #6
    Yamaha artisan Cutlass's Avatar
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    Well I cleaned up the jugs a little bit and quickly ran a 3 stone hone through the cylinders. I'm not real happy with the results but its not too terrible. I've received a few great PMs from a helpful member about some carb info. I'm now planning on doing the drill out the restrictors and add a main jet inline mod to help equalize the fuel pressure across the the 3 carbs.

  7. #7
    Yamaha artisan Cutlass's Avatar
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    Oh and since I haven't done so already, let me introduce myself. My name is Aaron. I've been fixing GM cars for 10.5 years now at a Cadillac dealership. I'm a Moderator on a different forum (LS1.com). I previously owned a 20 foot Concord Mach III GT boat with a 454 Mercruiser for 5 years, but sold it in May 2010. But I couldn't stay off the water, so I bought my GP1200 in August 2010.

    Well thats about it. Thanks for all the help so far. Keep the suggestions coming if you guys have any. I can return the favor if anyone has any car questions.


  8. #8
    I might have a stock bore oem piston for ya. I can check when I get home if you wanna go that route

  9. #9
    Flying Scotsman's Avatar
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    Quote Originally Posted by Cutlass View Post
    Oh and since I haven't done so already, let me introduce myself. My name is Aaron. I've been fixing GM cars for 10.5 years now at a Cadillac dealership. I'm a Moderator on a different forum (LS1.com). I previously owned a 20 foot Concord Mach III GT boat with a 454 Mercruiser for 5 years, but sold it in May 2010. But I couldn't stay off the water, so I bought my GP1200 in August 2010.

    Well thats about it. Thanks for all the help so far. Keep the suggestions coming if you guys have any. I can return the favor if anyone has any car questions.

    Welcome to the forum Aaron
    Why not bore her out and raise compresion.

  10. #10
    Yamaha artisan Cutlass's Avatar
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    Quote Originally Posted by tumblingtwerp9 View Post
    I might have a stock bore oem piston for ya. I can check when I get home if you wanna go that route
    Thanks, let me know!

    Quote Originally Posted by Flying Scotsman View Post
    Welcome to the forum Aaron
    Why not bore her out and raise compresion.
    Believe me, the thought keeps crossing my mind. Its not in my nature to BS a repair, I'm kinda a perfectionist. But I guess cost is holding me back. I wanna slap this thing together somewhat inexpensively. But it is becoming more and more difficult to resist the urge to do this the right way.

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