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Piston analysis

Discussion in 'General Tech' started by DTS, Aug 9, 2015.

  1. DTS

    DTS PR Addict

    I'd like to hear some opinions on this piston's failure. It's a Namura that came out of a craigslist special cr125. Stamped 54.94 meaning it is oversized. The bike actually started fine but ran poorly.

    A few things I'm trying to determine; the cause, if the piston is too large for the bore, and what size to re-instal.

    For the record, the cylinder is "ok"

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    Exhaust side

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    Intake

    image.jpg image.jpg


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    Ring gap

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    Standard 125 ring in same bore to show difference

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    Not sure if these are of any help, but here is side to side of the cylinder.

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    Front to back

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  2. Vet261

    Vet261 PR Addict

    Location:
    Valley City, Ohio
    I believe those are cold seize marks from improper warmup.

    Happens when you take a 2 stroke, fire it up cold and then start riding WFO right away. The piston expands quicker than the cylinder walls and and it "cold seizes."
     
  3. adam63

    adam63 PR Addict

    It's very hard to believe the cylinder is "OK". Will it run the engine again... sure. Will it ever seal up as designed... unlikely.

    I have seen re-plate companies leave the bore slightly on the tight side (.0005") and a slightly lean bike have these types of issues, oddly enough, with Namura pistons.

    Deff. looks like a rapid expansion problem to me also and no exhaust bridge lubrication? Does that cylinder have an exhaust bridge?

    At the very least I would send it back to the plater of my choice and have them inspect it and hone a few tenths of a thousandths out of it. The bore needs measured in more places than just at the last 1/4". Most likely they will want to re-plate it. Send your piston of choice with it so they know what clearance to set it at for best result. Once you put it back together be very careful with it until you have verified the jetting is correct.
     
  4. sturd

    sturd PR Addict

    Seizing on the intake side is either cold seize as mentioned (that's not that easy to do) or
    lack of oil in the gas. I'd bet lack of oil. How is the bottom end? They'll live longer than
    the piston on oil film left over from last tank but that isn't very long.
     
  5. DTS

    DTS PR Addict

    Poured out the tank tonight.. Didn't look like there was oil in it at all haha. Also, the previous owners should have their dirt bike ownership rights revoked. If it was my bike originally I would have never installed a junk namura piston. Basically my plan for this bike is just get comfortable doing rebuilds on my own. Good so far according to plan. This bike was very neglected.

    It does have a exhaust bridge, and I am surprised it is not cracked. Crank bearings and the big end bearing survived. So far I've rebuilt the entire water pump with new bearings and seals, and replaced the right crank seal. For the 1 min I did ride it, the bike was a fog machine.

    Jetting should good, and is comparable to my other 93

    Thanks for the replies everyone
     
    Last edited: Aug 11, 2015
  6. loneranger171

    loneranger171 PR Addict

    Location:
    Navarre
    Dans Bike Shop I can see it now. You're the first guy I'll call with service requests better brush up on ktm four strokes lol
     
  7. DTS

    DTS PR Addict

    Haha! The only thing I know about four strokes is that they don't take premix! Next year I hope you make it over to red bud for the vet fest. Also I'm trying to find a time I can meet you at beans
     
  8. loneranger171

    loneranger171 PR Addict

    Location:
    Navarre
    Hahaha maybe one day red bud just don't have time to take away for that long. Definitely want to meet up at beans I can use ur help getting comfortable on some jumps it's hell knowing 5 years ago I had everything dialed and now I'm getting butterflies b4 bigger jumps again and just rolling most of them
     
  9. DTS

    DTS PR Addict

    Corners are more fun anyways, I'm a slow poke when it comes to hitting a new jump
     
  10. loneranger171

    loneranger171 PR Addict

    Location:
    Navarre
    I agree nothing like railing a good turn... I'm not scared of jumping as much as I am not feeling the speed needed just yet. Always thinking too much when sometimes u just gotta twist it a little longer lol
     
    DTS likes this.

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