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Problems with CR250/265 big bore

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I had EG make a 265 big bore for my sons '03 CR250,and having major issues with it. Eric has done several cyl. tunning jobs for me in the past,but this is my 1st exp with a big bore. This is whats going on with it,a set of rings don't last anytime at all and when you they pull them out,you can see they are worn down about 1/2 way on the exhaust side,with new rings compression is only 170#s even on 1st build before ever cranking. Second,I am running a Rad valve and in no time at all it will completely suck one of the top pedals off and crack both carbon fibers. Next I tried a set of dual stage with a stock cage and they lasted only 15 mins. before splitting.'02-'04 CR250's were bad about chipping and splitting the stock reeds but have no problems after changing to aftermarket ones on our stock engines. I have sent cyl. back to Eric but wondering if anybody on here had any ideas.

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Was the bike running lean? Was there detonation? I had the ring thing happen to me once on my YZ 265 but it was due to dirt in the motor (air filter not clean:bonk:). Im not exactly sure what would cause the reed pedals to crack though, never had that happen to me

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I had EG make a 265 big bore for my sons '03 CR250,and having major issues with it. Eric has done several cyl. tunning jobs for me in the past,but this is my 1st exp with a big bore. This is whats going on with it,a set of rings don't last anytime at all and when you they pull them out,you can see they are worn down about 1/2 way on the exhaust side,with new rings compression is only 170#s even on 1st build before ever cranking. Second,I am running a Rad valve and in no time at all it will completely suck one of the top pedals off and crack both carbon fibers. Next I tried a set of dual stage with a stock cage and they lasted only 15 mins. before splitting.'02-'04 CR250's were bad about chipping and splitting the stock reeds but have no problems after changing to aftermarket ones on our stock engines. I have sent cyl. back to Eric but wondering if anybody on here had any ideas.

Where exactly are the rings wearing... just in a 5mm wide area in line with the exhaust bridge? Or right across the exhaust port? Did you drill the piston to get oil to the exhaust bridge?

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sounds like the rings are hitting the power valve.

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sounds like the rings are hitting the power valve.

+1 i had the same thought ... however i feel like EG would Cut into the power valve if he was going to bore a cylinder...

the next thought i have is maybe a crank issue as i know that when you bore out any engine it puts more fore on the exhaust side of the piston almost as if the crank is trying to push the piston out of the exhaust port

can you post pic

we need to see all sides of the piston including under the dome

This is all assuming your cylinder is "round" ..but again with EG i doubt this

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