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  1. #1

    Cracked Piston Causes

    What would cause the PTO cyl piston to crack? I just picked up another 750 SLT it has a cracked PTO piston. No sign what so ever of striking the head or other object in the cyl. The cyl bore is very smooth, no sign of scoring. Hour meter only reads 52hrs.

    I'm thinking water entry, or severe detonation from bad gas. Rod does not appear bent, so I don't think it hydro locked. The guy I got it from said it just suddenly lost power would only go 30- 35mph.

    I kind of want to just stuff in a new piston in that cyl and let'er rip. The cost as most know is around $150 for a single piston,rings, bearing, pin clips and gaskets. Think its worth just putting in one new piston? For about another $125 I can get a complete WSM top end kit off e-gay.


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Are the crank shaft and rod big end bearings all good?

  3. #3
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Right from the book.

    Cracked piston crown:

    Hydrolocked engine (water or fuel)
    Foreign material in engine


    Cracked/broken piston skirt:

    Excessive piston to cyl clearance
    Foreign material in engine
    Sharp ports
    modified engine
    hydrolocked engine (water or fuel)


    Make sure to index the crank before you go throwing $$ at the engine.

  4. #4
    bowsniper's Avatar
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    Where is it cracked? Any Pictures of it? How big a chunk? is it in 2 pieces? Did it crack vertically or horizonally? or the lands? or the top?

    If the lower skirt cracked, it could be piston-to-wall clearence.

    If the top exploded, it hydrolocked from water splashing in there possibly from water leak from the water rail if they didnt have the flame arrestor on it..

    some people will just take off the flame arrestor and put nothing on top of the carbs running it.. thinking its gonna increase horsepower. they they hit a wave and all the water inside the hull bounces around and dumps into the carbs, blows out the piston and it immediatley starts to slow down, as in your situation. no apparent damage,, just a crack and loss of power.

    That would be my bet. I always had water in my slt from jumping around and spinning.lol.

    That rod needs to be checked for being bent. If it's off 3 thousands of a inch. you wont be able to SEE that. then K-boomb after the new weisco piston set!

    Just my 2-cents. It would be interesting to really know how this happened so it wont happen to the rest of us. And what preventive measures we need to address..

  5. #5
    I dont have any pics yet. The piston is in two pieces cracked vertically. The top of the piston is black like a lean runner no wash marks near the intake ports.

    I do not have the crank out yet. I am now leaning VERY heavy to a 780 swap and putting in a rebuilt crank. What does indexing a crank normally cost?

  6. #6
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Quote Originally Posted by jsl750&slt750 View Post
    . What does indexing a crank normally cost?
    $0, just takes about 5 mins to do.

    Do a search for crank index.

  7. #7
    bowsniper's Avatar
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    get a crank and put the 780 top togther and youll have 30 more horse and be done with it. it would be capable of taking on more mods too since its new. you pick the piston size!

  8. #8
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Quote Originally Posted by bowsniper View Post
    get a crank and put the 780 top togther and youll have 30 more horse and be done with it. it would be capable of taking on more mods too since its new. you pick the piston size!
    ?????????


    Is he putting triple pipes or nitrous on it?



    750 = 80 HP

    780 = 92+ HP

  9. #9
    bowsniper's Avatar
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    Opps/ I knew better, I was thinking if he put on triple pipes on it..

  10. #10

    Broke Truck

    I'm on hold for now, my truck broke and I cant ride the ski to work

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