My son rides a 2002 RM85, and a couple of weeks ago, I noticed water flowing out of the radiator overflow tube. Upon checking the radiator, it was empty. A refill of water would only last 15 minutes or so, before it was empty again. When the bike is revved up, it pushes water out the overflow tube. We took the top end apart, and found a broken inner O ring between the cylinder and the head. I replaced the piston, ring, and all gaskets. On the first ride, the condition was the same. I pulled the head off again, and found the new O ring broken in the exact same spot as the old one. I've checked the cylinder and head for warpage, and all appears to be within spec. There are no sharp edges, or anything else that appears to be causing the O ring to fail. Has anyone ever seen this kind of condition? Thanks for any input.