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  1. #1

    97 SLX Pro 785 cylinder/piston help

    Hey guys. Basically new here. I signed up years ago but haven't been riding or hulking for a while.

    Anyhow here goes. My pro had been in mothballs for a while. I've decided to resurrect it.

    What initially had happened was that I, stupidly, over-torqued one of my exhaust manifold bolts. Needless to say it broke and when it broke it chipped off a part of the manifold mating surface in a way which made in unrepairable. No big deal. Bought a new cylinder and thought that should do the trick. It hadn't blown up, still had perfect compression it was just that the cylinder couldn't mate up entirely with the pipe manifold surface and caused a pretty significant water leak.

    So now with new cylinder installed I went to fire it up and to my horror heard "screech-schreech-schreech" as the piston went through it's cycle. I removed it again to make sure the rings were properly installed. They were. Tried it again go the same result.

    So I've read that I should install head before tightening the cylinder bolts to help align the cylinder. Might this help? Should I bite the bullet and buy a new piston too? I don't want to keep trying with the scraping/screeching in fear I will damage my new cylinder. This machine only has 100 or so hours on it. Should a fresh cylinder always get a fresh piston? And lastly, where is the best place to buy a new piston should I need one?

    Thanks to any and all who have advice or who can help.


  2. #2
    Tony's Avatar
    Join Date
    May 2007
    North Carolina
    Are you sure the noise is coming from the new cylinder? Remove the spark plugs and turn the motor over by hand.

  3. #3

    Join Date
    Aug 2006
    Homer Glen Il.
    yeah pull the pump to make sure the prop isn't rubbing on the wear ring

    just pull the 4 long bolts

  4. #4

    Join Date
    Sep 2006
    Houston Texas
    Agree with Tony, sounds like could be coming from somewhere else. Could you have maybe dropped something in the crankcase ? ( clip, part of the broken cylinder). Also you might want to check your clearance between the cylinder and piston.
    Harbor Freight sells a digital mike and bore gauge set. Cheap, but works. From my experience, it is usually best to install a new piston with a new cylinder.
    A stock bore 750 piston will work in 785. I have used WSM pistons in my limited race boat, and have had no issues.

  5. #5
    Hey guys thanks for the prompt responses. It is appreciated. Tony: That was the odd thing. I cranked it over and over by hand before firing it. I then hooked up the batt and ran it over with the plugs out and not a peep. It sounded perfectly normal with no indication of anything. It even sounded ok when I put the plugs in but left off the wires. I put a tablespoon of oil in and went to fire it. I KNOW nothing fell in the case. I was meticulous about that as I swapped the cylinder. And to update: Now I have the faintest vertical scoring marks on my cylinder. Nothing, I hope, that will make me lose compression but enough so I know something ain't jiving.

    So new piston time, right? I think that is the route I'm gonna take. Anyone got any other ideas? This screeching is 100% coming from the engine and now that I can see slight scoring marks I am pretty positive it's the cylinder I swapped. So Sloan, where should I get a new piston? WSM? Polaris? Someone else?

    Thanks again everyone!

  6. #6

    Join Date
    Sep 2006
    Houston Texas
    A standard bore piston form a SL 750 will work. There are couple of vendors on ebay that sell WSM pistons, fast shipping and good feedback. Also the WSM pistons are lighter and the motor should rev faster. How bad are the scoring marks ? You might want to run a quick hone in the bore.
    Make sure to check the clearance between the wall and the piston. And also check the condition of the upper rod needle bearing. If I remember had same issue, and was a worn\bad bearing

  7. #7
    Thanks Sloan! I'm gonna look for a WSM piston ASAP. And that's a great tip about the needle bearing. I'll be sure to check it when I pull the piston.

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