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  1. #1

    SLT 750 Cylinder & Piston service

    Hello, I've read this fine forum for a year or so and learned a huge amount.

    I have a 94 SLT 750 with a holed & broken piston (rear cyl). The bore is lightly scored, almost ridged along piston travel.

    The other 2 cylinders / pistons look perfect.

    Assume for now no other damage found. I assume the best way to proceed is to have the cylinder bored and install a matched oversized piston.

    Where would be a good place to have the cylinder bored and buy the proper sized piston, rings, clips, bearing, pin & etc?

    Thanks for all the help!

    PS - New triple outlet pump is installed and carbs rebuilt. I suspect this damage was from before the upgrades. Although the ski hasn't run really well the few times I've used it, it did run fairly well, top at 5200 to 5500 rpm with damaged piston!! It did run rough on acceleration and hole shot was weak. About 100 hrs. total on ski, most parts look new despite being 20 plus years old. Thanks again.


  2. #2
    BlueFishCrisis's Avatar
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    SBT has a cylinder swap option aside from a local shop boring the cylinders. Find a local shop that works on 2 strokes dirtbikes, 4 wheelers, etc. Usually around $60 a hole to bore. Check ebay for a new piston. There are several sources selling new pistons for around $70.

    Alternatively, you can look on ebay for a good used piston cylinder head combo......

  3. #3
    David Drkvampire2001's Avatar
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    John Zigler Owner of Rock County Jetski and Watcon.

  4. #4
    Thanks, I'll check with them both.

    Meantime, curiouser and curiouser - the piston is broken across the crown and down the sides through the pin bosses, it would not need too much more to fall in half. Plus a hole melted through on the top. The wristpin is bent and the needle bearings broken. Something very violent happened in that cylinder. Connecting rod looks ok. Cylinder itself has smooth ridged up and down but measures in or near spec. I think it's possible these are aluminum melted from the piston and left smeared on the wall. Piston scored and measures undersize.

    I'm going to have it honed then measure and decide how to proceed - possible I'll just put in a new piston and see how it does.

  5. #5
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by gbw View Post
    Thanks, I'll check with them both.

    Meantime, curiouser and curiouser - the piston is broken across the crown and down the sides through the pin bosses, it would not need too much more to fall in half. Plus a hole melted through on the top. The wristpin is bent and the needle bearings broken. Something very violent happened in that cylinder.

    Connecting rod looks ok. Cylinder itself has smooth ridged up and down but measures in or near spec. I think it's possible these are aluminum melted from the piston and left smeared on the wall. Piston scored and measures undersize.

    I'm going to have it honed then measure and decide how to proceed - possible I'll just put in a new piston and see how it does.
    That sounds like the cylinder was hydro locked with water ingestion. If the engine sucks in liquid water, the water will not compress and the piston comes to a very sudden stop.

    The critical question is whether the connecting rod is now slightly bent or if the lower end bearings were compromised by the stress. Hard to tell with the crankshaft still assembled in the engine.

    Check this as best you can, but I suspect you will just have to run the crank as it is and see if it holds together over time.

    Aluminum bits stuck to the iron cylinder walls can be removed using a specific acid which eats the aluminum but does not bother the iron.

    Have the cylinder measured before having it bored. If the cylinder is still within specification then you can just hone the cylinder walls and replace the piston and rings.

    The melted hole through the piston is symptomatic of the carburetors needing internal cleaning and rebuild. Go through the rest of the fuel system at the same time, everything from fuel tank to fuel pump.

    See my signature links for more info.

  6. #6
    When re-assembling, do I need to use sealant on the gaskets, and if so what?? - head, cylinder to block, exhaust manifold , carbs, etc.

    Thanks.

    PS - When I re-did the carbs I used Tygon yellow fuel lines w/zip ties. They are very soft and leaky, I'll change them to something stiffer.

  7. #7
    Click avatar for tech links/info, donation request K447's Avatar
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    Use proper fuel hose and proper hose clamps. I suggest reinforced (but not 'fuel injection' rated) rubber fuel hose, but other guys use the clear hose.

    I prefer the Oetiker style stepless/gapless ear clamps over the gear type hose clamps. The Oetiker hose clamps are single time use only but they provide a reliable and consistent clamping force on the hose. Cost per clamp is reasonable, once you have the required crimp tool.

    While some people do use zip ties, in my opinion these are just not safe for use on a vibrating, hot engine. A leaking fuel hose can create a very dangerous accumulation of gasoline fumes inside the enclosed hull.

  8. #8
    Tygon is supposed to be proper fuel hose but I don't like it. It softened and expanded and looks like it might kink easily. I'll use the rubber and better clamps, not worth taking a risk.

    Thanks, what about sealant on the gaskets - yes or no and which type? (Gasket surfaces are clean).

  9. #9
    ThrottleOut's Avatar
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    I have a good jug/piston/head combo if you're interested. Had 124 psi when pulled and is in good shape ready to install. Also have the wrist pin with needle bearing you could use with it. Shoot me a PM if you're interested.

    The cylinder base gasket will probably have to be replaced as they usually tear when removed. I'd replace the base gasket, head gasket and exhaust gasket if it were me. No sealant on those, but use threebond on the cases if you have to split them open.

  10. #10
    Below is a photo of the piston/pin/bearing. The crack extends down through the pin bore. The wrist pin looks bent because it is - slightly bent and flattened. Needle bearings were broken. Very violent event whenever it happened.

    I cannot say for sure if this occurred before or after I got the jetski - I only had a chance to ride it once and that was before I got the MFD working so I couldn't see the RPMs. I believe the piston cracked before the hole occurred. The only other time it was on water it showed slow acceleration and top end of 5200 - 5600 RPM and this is what I found. This is the rear cylinder, the others looks fine.

    I honed the cylinder and it cleaned up very nicely and in-spec as far as I could measure. Installed a new WSM std. piston and rings, pin and bearing. Head looked good.

    The big questions are the connecting rod and the crank. The rod piston-end was misshapped to where the bearing/pin could not be inserted. I cleaned it up and got a good fit, but clearly it was damaged. Don't know if the crank was damaged (knocked out of index or not).

    Anyhow, the jetski now shows 6230 rpm with 250 lbs (2 people) on it, acceleration is strong. There is a vibration at around 3000 rpm.

    All other mods are done - carbs, fuel pump and lines, oil lines, etc.

    We'll see how it lasts. If it fails again the decision will be to either scrap it or replace the short block, at some point it becomes not worth the effort / time/ expense. However, the remainder of the ski is near mint condition with fewer than 100 hours. Thanks for the help!

    Click image for larger version. 

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    I'll add, for what it's worth, and IMO companies should learn this: My grandson is interested in quads. Based on my experience with this machine Polaris as a company is excluded from consideration for anything, ever. Perhaps unfair but with the quality of the competition out there I want nothing more to do with them.

    I don't care that it was 20 years ago. This jetski should never have gotten to market. They knew of the problem within a year or two, and once it did they should have moved to correct this on all products produced regardless of cost.
    Last edited by gbw; 06-02-2015 at 10:56 PM.

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