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  1. #1

    787 Piston Backwards

    I just rebuilt the top end of my GSX-RFI, it starts and sounds good on the trailer, but wont run in the water. Upon further investigation I think I may have put the pistons in 180 degrees off. It has good compression. What does the small hole in the side of the piston do? It should be on the same side as the injectors? I hope I can salvage my parts. Thanks for any info or experience with this.

  2. #2
    PWCToday.com Is My Home Away From Home WFO Speedracer's Avatar
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    Re: 787 Piston Backwards

    You are correct that it will not run in the water if the pistons are in backwards,your parts should be fine but you will need two new piston pin circlips and gaskets of course.
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  3. #3
    resident guru
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    Re: 787 Piston Backwards

    That hole is supposed to cure a hot spot on the intake side of piston. WIth it backwards, it causes a major air leak and allows the rings to snag in the exhaust port. So get that motor apart and swap them around.

  4. #4
    PWCToday.com Is My Home Away From Home 1goofball's Avatar
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    Re: 787 Piston Backwards

    Wait before you pull engine. Take out plugs, shine a flashlight down hole and see which way the arrow on top of piston is facing. It should face towards exhaust. If can't tell, pull head and look.

  5. #5
    PWCToday Guru Ibreakstuff's Avatar
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    Re: 787 Piston Backwards

    These won't snap rings if in backwards?

  6. #6
    PWCToday.com Is My Home Away From Home Grumpy Old Guy's Avatar
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    Re: 787 Piston Backwards

    Quote Originally Posted by Ibreakstuff
    These won't snap rings if in backwards?
    Sometimes,sometimes not.

  7. #7
    Tech Guru PWCToday.com Is My Home Away From Home Watercraft Magic's Avatar
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    Re: 787 Piston Backwards

    Remove the rave valves and rotate the motor. If you see holes in the piston skirt from looking through the rave slots, you have them in backwards.
    Even if the rings did not break, they will have sustained damage from passing the large exhaust port and getting snagged, which usually damages the ring lands on the piston too. More damage will occur if you ran the engine much at all. You are lucky if you did not yet break a ring and ruin the head and clyinder walls too.

    The holes cannot be on the side that the exhaust port is on.

    The function of the holes is to help transfer intake charge from the crankcases to the combustion chambers and at the same time, The cool intake charge with oily gasoline, cools the underside of the piston crown and lubricates the upper rod bearings.
    Last edited by Watercraft Magic; 08-16-2007 at 01:02 PM.
    email Chris at Watercraft Magic cdofixr1@live.com

  8. #8
    Top Dog billman100's Avatar
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    Re: 787 Piston Backwards

    This is old, but I but I have the same motor. I put the pistons in backward, broke the rings, realized what I had done, bought new rings, installed the correct direction and now I only get the rear injector to work. I can switch the injectors and they both work, but from my reading only the front injector should work at idle. mine is reverse now. also can't get the front injector to fire at all. replaced the mpem, same result. can someone help before this gets parted out?
    Gen2 BOB JW850BB -Need balls to flip it-

  9. #9
    PWCToday.com Is My Home Away From Home WFO Speedracer's Avatar
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    Re: 787 Piston Backwards

    Quote Originally Posted by billman100 View Post
    This is old, but I but I have the same motor. I put the pistons in backward, broke the rings, realized what I had done, bought new rings, installed the correct direction and now I only get the rear injector to work. I can switch the injectors and they both work, but from my reading only the front injector should work at idle. mine is reverse now. also can't get the front injector to fire at all. replaced the mpem, same result. can someone help before this gets parted out?
    Actually my understanding is that every time the engine idles it is supposed to cycle to the other cylinder, what year is this ski ?
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  10. #10
    Top Dog billman100's Avatar
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    Re: 787 Piston Backwards

    99 gtx rfi
    Last edited by billman100; 10-18-2017 at 06:28 PM.
    Gen2 BOB JW850BB -Need balls to flip it-

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