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xj650 piston skirts

Discussion in 'XJ Technical Chat' started by joejr2, Oct 9, 2018.

  1. joejr2

    joejr2 Active Member Premium Member

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    I recently pulled the cylinder jugs off the 650 maxim engine I have been working with. At top dead center
    #1 and #4 pistons are up and I noticed that #4's skirt extends down at the front and back, while #1's skirt
    extends down at the back only. The front is cut off even with the cut below the wrist pins. Is this abnormal ?
    The horizontal cut in the front feels like it was not hacked off, but machined like that. I have not seen the skirts under #s 2 and 3 yet, as those two are down at TDC.
     
  2. Toomanybikes

    Toomanybikes Well-Known Member

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    Maybe someone has had just one pisto replaced. What year is the bike?
     
  3. joejr2

    joejr2 Active Member Premium Member

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    The bike is an 82 650 maxim. Upon closer inspection that skirt on #1 piston looks cut off. Under a magnifying glass
    I could see that it was cut and filed. If that piston was modified in that way, wouldn't it be badly balenced ? My next
    question would be; " can I replace just one piston ? or do all 4 have to be replaced as a set ? "
     
  4. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    You can replace just one, but you'll have to measure the bore to make sure that it can still accept a standard piston.
    Running a piston that has had only one skirt removed will lead to uneven bore ware. Hopefully the engine was not run long with that badly modified piston in it.
     
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  5. joejr2

    joejr2 Active Member Premium Member

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    I measured each bore at 4 evenly spaced locations on 4 levels with a bore gauge and digital caliper and found all the diameters to be
    between 62.9 and 63 mm, no damage that I could see. I went back and measured each bore twice just to make sure. I think the guy
    that modified that piston must have dropped it, broke a skirt and tried to salvage it. I bought it as a forsaken project. An engine in a frame
    on a dolly and the rest of the parts in boxes.
     
  6. Colin 85 700

    Colin 85 700 Active Member

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    Im sure chacal, Len can help you find a piston... Theses duded are great
    Send a message to XJ4ever, top right of most pages, the engine icon!
     
  7. Colin 85 700

    Colin 85 700 Active Member

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    I'd check the cylinder for out of round and "barrelling" where it wears in the middle of the cylinder if it had a a short skirt piston, if it's in spec, ride it with a used piston! :)
     
  8. XJ550H

    XJ550H Well-Known Member Premium Member

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    upload_2018-10-11_19-56-30.png
    upload_2018-10-11_19-57-13.png
     
  9. a100man

    a100man Well-Known Member

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    Stuff a standard piston in there, new rings if you can stretch to it otherwise whatever you can find. These motors are pretty tolerant to wear I reckon, so unless you plan on going racing...
     
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  10. joejr2

    joejr2 Active Member Premium Member

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    I used that same pattern for measuring cylinders as xj550H. Except that I divided the cylinders into 4 rather than 3 levels and measured across each level
    at 4 places with a bore gauge and a digital caliper. I believe it gave me a more accurate picture of the internal topography of each cylinder. I was lucky to
    find that all the cylinders were within those same service specs that xj550H posted.
     
  11. joejr2

    joejr2 Active Member Premium Member

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    Ok, so now I,m ready to hone the cylinders, now that I know that they are within specs. I want to use an abrasive ball brush hone to cross hatch the cylinder walls
    rather than using stones that might remove too much material. My Question is, what diameter should the hone be for a 63mm + - cylinder and what kind of lube should I use ?
     
  12. Colin 85 700

    Colin 85 700 Active Member

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    A spring loaded bar hone in a drill is cheaper, more universal for diameter and easier to use.
    Also a bar hide will help to "square" the cylinder if it's a bit out.
    Either a 400 grit stone or my favorite, wrap the hone in fine emery cloth for a faster break in.
    Once you see a nice 60 degree cross hatch, stop, don't go crazy, should only take a few seconds.
    60 degree meaning the crisis cross should be roughly 30 degrees from horizontal, left and right hatch. Giving a 60 degree appearance in the horizontal corners of the "diamond" shape of the hatch.
    ><><>< <-- 60 -->
    Lube with 2-stroke oil.
     
  13. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Ther reason for measuring in thirds is because of how the piston rocks as it runs in the bore (due to the action of the crank on the con rod). You're probably fine they way that you measured, but there is a chance that you missed an out-of-spec condition midway up/down the bore.

    2 5/8 " ball hone, 240 grit. Lube with motor oil.

    Flat stone hones take a bit more practice to use, and really aren't necessary if all you need to do is break the glaze when installing new rings. I tend to save the stones for situations when I can catch a nail on an imperfection in the bore and honing will still keep it at the standard size.

    As with many things having to do with putting a finish on a part, personal prefrence is always in play. Many ways to get the same result.
     
    Last edited: Oct 26, 2018
    chacal likes this.
  14. joejr2

    joejr2 Active Member Premium Member

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    I measured the cylinders with a bore gauge across the diameter in 4 places and at 4 separate levels. 1/2" from the top, 1/2" from the bottom and 2 places in the middle, dividing
    the cylinders into thirds between the top and bottom station points. Everything was within the book specs, taper, out of round etc. One thing I noticed was a vertical scratch on
    the wall of cylinder #1 lined up with where the circlip would be except that the circlip for The #1 piston was missing. I was afraid that the clip could jam something so I used
    a flashlight and a mechanics mirror to check all around the crank and found nothing. Likewise pulled off the oil pan, clutch cover, and shifter cover and also nothing.
    The oil was drained and still no clip. I turned the engine on it's side looked up into the oil pump area and still nothing. am I safe to assume that the clip settled down somewhere under the
    gears and will probably stay there since it's heavier tha oil. The engine rotates smoothly and freely with no rubbing or grinding. Am I wise to ignore the missing clip since it doesn't
    SEM to be stuck in anything ?
     
  15. Polock

    Polock Well-Known Member

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    this is the wrist pin clip, right?
    if the clip was on when the piston went into the bore, i don't think it could get out of there, except in microscopic dust. i don't think the wrist pin comes down past the cylinder liner for it to get out.
    sounds to me like someone just put it back together and kept their fingers crossed
     
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  16. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    A previous owner masterpiece............
     
  17. joejr2

    joejr2 Active Member Premium Member

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    Also, one of the bottom skirt extensions in the front of #1 piston was cut off flush with the skirt level under the wrist pin.
    I was able to buy a replacement set of pistons on ebay. Since the jugs were in spec, and these new pistons are in good shape,
    I'm hoping for better compression when I get the whole thing back together.
     
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