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Thread: Rebuild Gone wrong

  1. #1
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    Rebuild Gone wrong

    my friends atv got rebuilt by 3rd party and in the process the head got smashed (valves into piston) and now its burning oil with less than a tank of fuel through it.

    they installed .5mm O/S piston, smashed the valves into the piston, then pulled it apart again and installed a new head.

    i pulled it apart and from what i can tell with my vernier the bore was too tight (it has clearanced itself) the head looks good.

    thinking the best bet is to go to the next size up piston and try again.. thoughts?

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    Now i have a JEEP I am getting good at fixing things lol

  2. #2
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    If that was a brand new piston with less than a tank of fuel run through it, I would agree that there is probably a clearance issue. A seized wrist pin can also cause significant piston scuffing as well.

    But no need to guess. Before doing anything, I would find the piston to cylinder wall clearance spec and check it with that piston to see what you get.

    Edit -a couple more thoughts: I see that bore was honed...but was it actually bored out for the oversized piston? I assume it was but you never know. Irregular or severe piston wear can also occur from an out-of-round bore. The top of the piston and ring lands appear to be untouched where as the skirt has significant scuffing.

    Also, IMO, you need a proper micrometer to measure the piston OD and the bore ID. Vernier calipers are handy for quick measurements but their accuracy can be questionable when you start getting down to a couple of thou.
    Last edited by CTD NUT; 01-03-2017 at 03:14 PM.
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  3. #3
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    ok i measured the cyl with micrometer and its really quite close to the 85mm spec not smaller. within .0005"

    so i looked at ring specs again, the top ring was loose and the second ring was on tight side of spec. turns out they put the rings in the wrong order

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    So, the piston to bore clearance is good?

    I assume the ring end gap specs are a little different for the top and bottom compression rings and the rings were just reversed? How would that explain the scuffing of the piston? And excessive oil burning? If the piston to wall clearance is good, the oil control rings should do their job even if the compression ring end gaps are out of whack.

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    yes the top ring is rounded, the second ring is basically a scraper, they had the scraper on the top and the rounded one in the middle. also the oil rings were not separated as far as it should be, only about 1/2" apart, should be 1" each side.

    the piston clearance was on the loose side but still in spec.. the piston measures around .0015" worn down from manufactured specs, and looking at it that seems about correct.

    i have googled it and it could cause the oil burning as they are specific rings for each groove.

  6. #6
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    Interesting. Glad you figured it out.

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