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  1. #1
    boomanch1967's Avatar
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    1996 SLT 780... never seen a motor do this before

    Pto PistonClick image for larger version. 

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    A ton of debris as anyone could well imagine with half the piston gone. Jug is scored, will be getting machine bored over, and proper bottom end work. My question is what could have caused this.
    Also this was an auction machine, so we had no prior history on it, and it popped on the hose, was just turning idle down was ideling at 3500, so turned it down.
    Started machine back up, turned water back on, idle was smooth, bumped the throttle 3 times, and rattle rattle... and you see what the rattle rattle was. Probably never saw above 3.6-3.7K while on the hose. Ran for maybe 20 seconds, turned idle down, started back up after a minute or two... 3 burps on the throttle and done...
    Compression before melt down was from PTO to Mag, 110, 110, 120... so it was out of what I would call a healthy engine spec anyway.
    Wash was good on Center and Mag, PTO was coated in Alum... Plugs before melt down were the correct shade of brown... No obvious damage to cases, I had checked crank index prior to compression test. Center TDC, same height measurement PTO and Mag.
    Any ideas would be appreciated, don't want to put this thing back together to do it all again.
    BTW- Only damage to piston crown was edge damage where the skirt debris was ejected out of the exhaust, and you can see deformation as the piston smashed any debris on the port edges. No erosion, and no spray of alum on the plug like a burning down piston.


  2. #2
    Moderator beerdart's Avatar
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    Excessive piston to cylinder clearance.

  3. #3
    boomanch1967's Avatar
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    Quote Originally Posted by beerdart View Post
    Excessive piston to cylinder clearance.
    Thanks, yep it is major league excessive now. The motor was ready for a rebuild anyway with those numbers... The cyl at the base is fubar'd, further investigation, and knowing the starting compression numbers 110, 110, 120, Mag piston is brand new... so somebody was in here, backyard blow and go for the auction. Lots of combustion trails below the rings...

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