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  1. #1

    Low Compression Rebuild

    Ok so I'm new here.... Hello all!

    I have recently acquired 2 1995 Polaris 780SLX for a great price _ FREE!!!

    I pulled a compression test on the first ski and I was 110 100 100

    The engines lacked power and the compression being low obviously mandated and overhaul-- based on my prior 2 cycle experience. I am a first time ski owner.

    I happened to receive a shop manual with my great find and have removed the engine and disassembled and inspected.

    Here is what I found.....
    Slight carbon below the rings
    glazed cylinders
    but overall in good shape.

    #1 71.35mm bore 71.23mm Piston and rings within spec.
    The bore is .05 over but the rings are fine .... indicating piston wear?
    Piston clearance is out of spec.
    #2 and #3 are the exact story just slight diff measurements increasing towards #3

    What do I replace piston rings? Do I hone? or bore?
    Please advise!

    Thanks in advance!

  2. #2

    Join Date
    Sep 2009
    Welcome yo the Hulk. I think you answered your own question when you stated the pistons were out of spec.

  3. #3
    bowsniper's Avatar
    Join Date
    Aug 2008
    Welcome to the hulk!

    Thats a great deal. free. I'm not familiar with the 780's. One of the guys on here will chime in with a solution for ya.. Are u looking to keep both skis? rebuild both?

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada
    Welcome to the Hulk

    If you haven't seen it yet, click below for useful info

  5. #5

    Parts to order

    Quote Originally Posted by cwag911 View Post
    Welcome yo the Hulk. I think you answered your own question when you stated the pistons were out of spec.
    Well the pistons being out of spec with the clearance to the cylinder like you said seems obvious.

    Here's the dilemma....

    Pistons available are STD 71mm? 71.25mm? and 71.5mm?

    Cylinders are 71.33mm- 71.35

    Piston to Cylinder clearance is: .09- .13mm and limit of .20

    So should I hone/deglaze my cylinders or replace the cylinder sleeve?

    Should I use oversize pistons? or Std?

    Any help would be appreciated......

    I'm hoping to successfully rebuild both engines.

    I already know that I need a stator and cdi for 1 of the ski's.

  6. #6
    FLjoyrider's Avatar
    Join Date
    Jul 2007
    If it were me, I would have run it at 100psi till it blew up all the way. 100psi is enough to run decent, as long as they are all within 10% your fine.. your just under 10%. I'd give the cyl's a slight hone stick some new rings and circlips in there and ride it. If you want to have a fresh top end go with the 71.5mm and bore the cyl's out. theres no need to resleeve yet, you still have one bore to go.

    I've found that most of the time even if ur compression is low (above 90psi will still run decent) the lack of power is a fuel delivery problem or reeds or pump problems look into those as well.

  7. #7
    You would be looking at about $350-$380 for boring and top end w/gaskets. Might want to put new seals in it also if it has high hours on it. Other than that if you want to make it this season just do a light hone, new rings, clips, and top end gaskets and go. Spend the money and time in the off season. Only sleeve or find other used cylinders unitl you can't bore it any more. More than likey you can get used cylinders for less than sleeving. WSM pistons go to 1mm over you you have 2 bores left.

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