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Thread: Cracked Piston

  1. #1
    Moderator DrewNJ's Avatar
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    Cracked Piston

    A friend of a friend who rides with our group read on the boards about the timing mod that most of us do cutting the rotor. His only other motor mods are an intake (he thought it was an old R-rated but it looks like a Riva to me).
    He cut the timing wheel himself at the beginning of the season and said that it's the best mod he's ever done, ski flies. All was good.....
    While riding with him on Sunday he pulls up to me and says the ski is down on RPM's...like 7200 vs. the 8100 he was seeing. We swap skis and sure enough he's way down on rpm's. Pull the seat and I'm greeted by a hull full of smoke and oil all over everything. Looked to me like it blew out the front valve cover gasket seal at the cam lobe area. OIL EVERYWHERE. Shut it down and towed it back. He dropped it off at my house and the first thing I did was a compression test. 130# on 2,3,4....#1: ZERO.
    Didn't put a caliper on the rotor yet but one thing I CAN see is that it looks like the pin is totally buried in the "cut section". There is a LOT of material missing. he said he used a "ruler" to measure the cut...

    This is my first time pulling one of these motors apart and I have to say that they are pretty simple....Not really a lot going on inside there. So, we've got parts on order and we'll get it back together.
    Here are some pics of the piston. I cleaned it up to give him as a paperweight...

    Moral of the story guys is you really want to be SUPER carful with the timing on these boats. I believe there is a bit of play for timing increase but just be careful.
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  2. #2
    My name is Sean and I am addicted to STXs smokeysevin's Avatar
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    if its a 250 with 250 pistons I have a set he can have if he pays shipping, all 4 are good

    Sean

  3. #3

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    So,
    Detonation is the culprit?
    Would the octane have made any difference?
    Any damage to the cyl bores....or is he extremely fortunate to have caught this early?

  4. #4
    Moderator DrewNJ's Avatar
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    Damn....Thanks for the offer Sean, but he already has some new one's coming...

    No damage to the cylinder bores at all, not even any marks on the sides of the piston and the even the wrist pin looks good with no marks or unusual coloring. I think all the timing just beat the piston top to death.
    I've seen this in the drag car world and it always seemed to be related to to much timing.

    93 octane run all the time as far as I'm told.

    I just wanted to put this up because there seems to be a LOT of questions about timing recently...Lots of questions about timing that is WAY above the common 250/6* mod...
    Hate to see people burn up these motors.

  5. #5

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    That's what it looks like, alright.
    Thanks, I've avoided messing with timing(it's a huge stresser IMHO) but it's always interesting to see/learn from what comes of the engines that get REALLY advanced

  6. #6
    Take the time to smile sirbreaksalot's Avatar
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    I have seen 07's do this WITHOUT timing mod

    normally piston #1 on the skiies I haves seen aswell
    Jet250 broke #1 on his ski , but was due to lack of fuel,
    So consider if someone rides their ski WOT while getting fuel surge!! ( but does not feel the surging because of the rough water)

    this will also induce detonation , thus assisting in cracking the piston ( i bet this is another reason they/kawi went to a high compression piston ( thicker piston crown material )

    So yes timing does crack and destroy pistons, but remember a 07 ski has the least compression and timing of all and they run the richest and can just do it out of bad luck charlie

    Waz

    my ski has run for approx 65hr on pulley kit 1-1 with 8ish deg timing sofar ,on crappy AUS fuel and still strong as day 1 , so many factors come into engine failures, and i spin it to 8250 -8050 allday.
    ( but with correct AFr's)

    not saying its the right thing to do ,just saying that too much timing alone on a NONE pulley 07 boat would have to be VERY wrong to do this alone if at all !!

  7. #7
    nicjak's Avatar
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    and mine ran same as waz above but melted the top off no.2 piston, so at the end of the day lots of factors can cause it. gone back to stock timing for fear of doing it again.

  8. #8
    Hooah_Killer's Avatar
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    Yeah before they put my engine back I had them install an oem rotor so until I get Dan the man's shop to install lambda, hdfp and ext fpr it's all stock timing.....even if John says it's no problem

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