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Did a hare scramble couple weeks ago did the race loaded the bike up everything seemed fine. Drove 5 hours to Arkansas this last weekend get down there no compression and my one kick bike won't start. Pull the top end off at camp and find this. IMG_1491233368.680458.jpgIMG_1491233384.319573.jpgIMG_1491233408.973964.jpg

And anyone have any pointers where to take the head and cylinder to get fixed?

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Something came loose in there and messed up the piston and rings not to mention cylinder. But Im sure you already knew that.... Maybe a circlip or the spark plug electrode broke off. Ive had good luck with Millennium Technologies for new cylinder chrome

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Probably detonation same thing happened to my cr 250 looked almost Identical. Were you running a fuel lower than93 by chance? And your cylinder might need welded. I always sent mine to powerseal USA and it cost me 260 for weld and replate. As for the head I would take it to a machine shop to get resurfaced

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Ya 91 I believe.

What kind of bike is it? How long have you owned? You might need to mix race gas with your fuel

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It's a 15 ktm 300 xc. All stock besides exhaust. I rode it for multiple hours after I bought it. Then just re did the top end it probably had 8-10 hours on it when this happened.

I'm not an expert but that looks like detonation to me so you might need to run half race gas or get the head machined for a little less compression. Just my .02

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The sides of the piston show a seizure. The head and top of the piston show something solid banging around in there. Check your airbox, filter and hoses to see if there was a way for a rock to get in.

I've verify that the ignition timing is right. Are you running crappy gas or gasohol?

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Dumb on my part also, tried pull starting it at first after it didn't want to kick over. Pulled for about 30yards started briefly and died. Then I tore into it. So that's probably where the seizure is from

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I second the detonation option.

Is it possible you used a thin gasket when you did the top end?

This will create higher compression, and with the low-octane fuel might cause detonations and this kind of damage. 

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If there is up and down play in your main rod bearing your crank needs rebuilding, some of the bearings have started breaking up.

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Hmmm, a lot of wild guesses here guys. If you don't know, telling him "more octane", "thicker headgasket" is no help.

Your rod bearing failed, and it does look like detonation may be the cause (hard to tell from photo), but what caused the detonation?

There is no headgasket, just an "O"ring I believe. What was your squish set to? It is set with the base gasket thickness. 

Was the wristpin and bearing replaced? It should be with every new piston.

Bad news is you need a new bottom end as well as a piston, cylinder replate and head cleanup. 

All the bits of metal are down in the crank and rod big end bearing. Need to clean the pipe out too. 

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Cases are split and on their way to being put back together soon. Deff rod bearing failure. I bought A new crank assembly and bearings. I've always replaced the wrist pin and needle bearing with every new piston. Didn't necessarily set the squish but put the same thickness of gaskets on as I took off. Will be setting "proper" squish this time

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That's a rod bearing most likely (less likely a main bearing) going out, it's coming apart and up through the transfer ports.  Time for full rebuild.

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That's what I thought but no one else brought it up. The rod bearing was definitely toast. I didn't know if it was from the heat or it just went. But she's done been split and on her way back together. Messing with the tranny, shift forks, drum and all that trying to get the forks in the right groove....having a hell of a time

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On 4/13/2017 at 11:22 PM, Missouri Railroader said:

(rod bearing) That's what I thought but no one else brought it up. <SNIP>

Read the post from April 10! 

 

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