KX85 Cylinder damage

exbmxmx
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exbmxmx
Edited Date/Time 3/23/2018 9:45am
My friend has a 2015 KX85 that was just torn down for a top end job after 20-hours of use. The bike has 100-hours on it right now and the previous top end rebuilds never had this type of damage. Is this caused by too little squish? or detonation? I believe he blends 50:50 with 93-octane and 112-octane and mixes at 32:1 Motul 800. The carb jets are 145 main and 40 pilot. This is the first rebuild after installing a crank and the engine builder now says after looking at the cylinder picture that he should run straight 112-octane. I'm not sure that is the solution, just a band aid fix. I do not know what the squish was set at but the bike is completely stock using OEM gaskets on the rebuild.

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exbmxmx
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exbmxmx 3/22/2018 11:42am
3/22/2018 11:42am
Here's the cylinder head.

Paw Paw 271
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Paw Paw 271 3/22/2018 12:07pm
3/22/2018 12:07pm Edited Date/Time 3/22/2018 12:08pm
The cylinder needs work. The head does not show any big bad signs of detonation due to a squish issue or low octane fuel.
I wonder if may be an incorrect head gasket was used that may have caused the damage shown in the photo.
( Like maybe a 105 head gasket?)
What does the top and bottom of the piston look like?

Paw Paw
Paw Paw 271
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Paw Paw 271 3/22/2018 7:05pm
3/22/2018 7:05pm
No detonation there and no signs of a squish problem. Every thing still points to the wrong head gasket used, IMO.

Paw Paw

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Tryhard
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Tryhard 3/23/2018 4:16am
3/23/2018 4:16am
yeah i would definatly say for 20 hours , the head and piston look great , the 50:50 mix and jetting looks spot on without carbon on the postion nice even burn . I would definatly look into the head gasket aluminum will break down under the high head and tempature . yeah i 2nd the head gasket , i sould also check the head and just double check the mating surface is true .
exbmxmx
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exbmxmx 3/23/2018 9:45am
3/23/2018 9:45am
He used the correct part number for the OEM Kx85 head gasket, but I can see why you would think that because that looks like a possibility to me too.

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