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  1. #1
    Frequent Poster metal4130's Avatar
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    Yamaha gp1200 piston specs.

    I am in the process of rebuilding my fully ported gp1200 race motor and I am having some issues with some of the piston specs from the manual. I checked all 3 of the cylinders with my bore gauge and they are all with in the specs of taper and out of round-ness that Yamaha gives in their factory service manual. I am just going to give thoese a good hone and be ready to go. This is where I ran into my problems. I started to measure the piston skirt diameter and my measurements seem to be off about 3 thousands in respects to Yamaha's specs. The spec that Yamaha gives is 3.3032 to 3.3040 inches for the diameter. On piston one I get 3.300 on two I get 3.301 and on three I get 3.300. My concern is that this is a significant difference from what Yamaha reccomends and I just don't see how all 3 of the pistons could be off by almost equal amounts. I am using all Mitutoyo mics. to measure this, and I double and tripple checked so I know these measurements are spot on. My question to you is 1.) have you ever heard of this before? 2.) should I replace the pistons?
    Thank you for your time!
    2 97 GP1200's
    91 Freestyle Superjet
    98 Semi-stock Roundnose

  2. #2
    Frequent Poster metal4130's Avatar
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    Re: Yamaha gp1200 piston specs.

    Nothing Harry?
    2 97 GP1200's
    91 Freestyle Superjet
    98 Semi-stock Roundnose

  3. #3
    Tech Guru resident guru
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    Re: Yamaha gp1200 piston specs.

    Quite frankly, I'm surprised that your pistons are showing only .003 of collapse. We very often saw running pistons with .006-.008 of collapse. If you are building a high performance platform, definately replace the pistons.
    Respectfully, Harry Klemm


    Quote Originally Posted by metal4130
    I am in the process of rebuilding my fully ported gp1200 race motor and I am having some issues with some of the piston specs from the manual. I checked all 3 of the cylinders with my bore gauge and they are all with in the specs of taper and out of round-ness that Yamaha gives in their factory service manual. I am just going to give thoese a good hone and be ready to go. This is where I ran into my problems. I started to measure the piston skirt diameter and my measurements seem to be off about 3 thousands in respects to Yamaha's specs. The spec that Yamaha gives is 3.3032 to 3.3040 inches for the diameter. On piston one I get 3.300 on two I get 3.301 and on three I get 3.300. My concern is that this is a significant difference from what Yamaha reccomends and I just don't see how all 3 of the pistons could be off by almost equal amounts. I am using all Mitutoyo mics. to measure this, and I double and tripple checked so I know these measurements are spot on. My question to you is 1.) have you ever heard of this before? 2.) should I replace the pistons?
    Thank you for your time!
    Unless otherwise stated, responses to all forum posts are based on my personal experiences working within the PWC industry, and developing high performance PWCs since 1987.

    www.groupk.com

  4. #4
    Frequent Poster metal4130's Avatar
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    Re: Yamaha gp1200 piston specs.

    What causes them to collapse? The pistons show almost no signs of wear at all. Do you see this sort of thing a lot?

    Thanks for the responces!
    2 97 GP1200's
    91 Freestyle Superjet
    98 Semi-stock Roundnose

  5. #5
    Tech Guru resident guru
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    Re: Yamaha gp1200 piston specs.

    Every piston in every PWC experiences skirt collapsing.... some engines experience more of it than others. In all cases, the piston "looks" completely normal.
    Respectfully, Harry Klemm


    Quote Originally Posted by metal4130
    What causes them to collapse? The pistons show almost no signs of wear at all. Do you see this sort of thing a lot?

    Thanks for the responces!
    Unless otherwise stated, responses to all forum posts are based on my personal experiences working within the PWC industry, and developing high performance PWCs since 1987.

    www.groupk.com

  6. #6
    ScottD
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    Re: Yamaha gp1200 piston specs.

    Harry do you ever strip the motor down just to check clearance.
    Or can you tell by piston slap stethascope or other methods.
    SCOTT............

  7. #7
    Tech Guru resident guru
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    Re: Yamaha gp1200 piston specs.

    Scott,
    The only way to correctly check the clearance is to remove the cylinder....we do it often. There is no way to accurately measure the clearance with the cylinder still mounted.
    Respectfully, Harry Klemm


    Quote Originally Posted by ScottD
    Harry do you ever strip the motor down just to check clearance.
    Or can you tell by piston slap stethascope or other methods.
    SCOTT............
    Unless otherwise stated, responses to all forum posts are based on my personal experiences working within the PWC industry, and developing high performance PWCs since 1987.

    www.groupk.com

  8. #8
    ScottD
    Guest

    Re: Yamaha gp1200 piston specs.

    I assume pistons collapse mainly due to heat
    Do the aftermarket pistons retain size better than OEM
    I have seen a good running motor that had 85.5mm bore prox pistons and .011 clearance.
    I wouldnt put it back together with that clearance but would not have known without inspection.
    Some stock motors run for hundreds of hours with no problems but if you tear down one of those motors you still wont find original bore clearance's.
    SCOTT..............

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