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  1. #1

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    GP760 Rear Crank Seals (Oil Seals)

    I would like to replace the rear crank seals (oil seals) on my GP760. I believe that there are two oil seals at the rear of the engine and one at the front. Is it possible to replace the rear seals without tearing apart the engine? What is the easiest way to replace the rear seals?


  2. #2
    You have to tear the motor apart.

  3. #3

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    I am going to pull apart the engine and change the front and rear crank seals. I would like to do this in the most simple way possible as this engine had a fresh top end rebuild 30hrs ago. Before i start this task i have several questions. 1) Can i split the case without removing the flywheel. 2) Are there any special tools I would need to complete this project. 3) As a result of taking the engine apart what other seals in other parts of the engine will defiantly need to be replaced. Can i just re-use all the other engine seals or do some always need to be replaced at the time the engine is reassembled. Should i get some type of rebuild kit with all the engine seals or can I simply change the 3 crank case seals then reassemble as that is my main problem. 4) How do the oil seals get reinstalled ie: what is the proper way to orient them in the case.

  4. #4
    You can split the case without taking off the flywheel, but I don't know how you're going to get the seal on that end of the crank without taking the flywheel off.

  5. #5
    showmepro1200's Avatar
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    You have to take off the flywheel and the pto-side coupler, but you may be able to get away with replacing seals without tearing it entirely apart. I've done it with a 701 Yamaha (twice). You flip the motor and remove all the case bolts to expose the crank from underneath, and lift each end of the crank just enough to get the seals out and new ones on. Then you clean the mating surfaces up and use Yamabond (or equivalent) and put it back together with proper torque procedures.

    As far as a 760 goes I would think it can be done - but I've no experience with it - check around before attempting. With my 701s it was just a maintanence thing, they didn't need anything else done. Considering things happened to blow the seal, you may want to do a full tear down on your motor for good measure.

  6. #6
    Yamaha artisan Cutlass's Avatar
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    Quote Originally Posted by showmepro1200 View Post
    You have to take off the flywheel and the pto-side coupler, but you may be able to get away with replacing seals without tearing it entirely apart. I've done it with a 701 Yamaha (twice). You flip the motor and remove all the case bolts to expose the crank from underneath, and lift each end of the crank just enough to get the seals out and new ones on. Then you clean the mating surfaces up and use Yamabond (or equivalent) and put it back together with proper torque procedures.
    This should work just fine.
    So OP...you need 3 crank seals, maybe a front cover gasket, maybe a oil pump gasket, maybe some intake, carb, exhaust gaskets (depending on what needs to come off to get the engine out of the ski), loctite, sealer for the crankcase (threebond 1211), probably some tip ties/clamps for the fuel and oil lines you remove, tools to remove the flywheel and tools to remove the PTO coupler.

  7. #7

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    Pulled the motor today and broke it down all the way to the crank case. Got the flywheel and coupler off too. I have ordered new seals for the entire engine. Front crank seal looked to be intact while rear seals were definitely shot. Although I am going to replace the crank seals I am seeking to understand what caused the crank seals to fail in the first place. Maybe they were old and worn out but as some have already suggested to me there could be a larger issue which caused the crank seals to fail. As I was disassembling the engine I took detailed pictures of all engine components but am not exactly sure what parts of the disassembled engine I should examine and what I should look for to understand what caused my initial problem. I have attached pictures to this post of the reeds because they look clean to me but I don’t know what bad reeds would look like. I have also attached pictures of the underside of the head and the cylinders as something looked a little strange up there to me. There was no scoring in the cylinders as the engine had a top end rebuild 30hrs ago. But I did notice that cylinder 1 (front) was completely covered in oil and while cylinder 2 (rear) was covered with less oil. I also did a cursory examination of one of the carburetors and it seemed like the diaphragm was in ok shape but not good enough shape to make me comfortable. I have a rebuild kit for the carbs and will likely be installing that before starting the engine again. I should also mention that before I started this project the ski was getting in to a "lean runaway" condition when it was started on the trailer. Any suggestions about the initial cause of this problem would be greatly appreciated? I can post some more pictures of the engine if there is another area of the ski that should be examined to understand this problem. Also, how do I check the condition of the crank shaft since it is completely exposed at the moment and easy to access?

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    Cylinder 1 on left in pic above
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    Cylinder 1 in pic above
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    Cylinder 2 in pic above
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  8. #8
    showmepro1200's Avatar
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    And get the coupler and flywheel off BEFORE you flip the motor too

    Nevermind - I see you've already gone into it.

  9. #9
    Yamaha artisan Cutlass's Avatar
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    The crank seals were probably just old, then they leaked air into the crankcase causing the lean runaway and the leaner appearance on the head. Remember, this is a 2 stroke. The engine has to be 100% air tight. The crankcase is part of the intake "manifold" and any leak into the crankcase will lean out the air/fuel ratio. The rear crank seal is near the top of the list of common air leaks on these PWC engines. Make sure to pressure test the crankcase when you are done reassembling the engine. There is a good how-to sticky thread somewhere around here.

    Reeds look fine. shine a flashlight through them to make sure all the pedals are sealing.

  10. #10
    BLACKandSILVER06GPR's Avatar
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    Also when assembling the top end back together the gap of the rings should be checked as well to play it safe since everything is apart. I don't know the proper gap but maybe someone can chime in if they know it. I feel like it is 0.2 - 0.4 mm but don't hold me to it for both the top and bottom rings.

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