Jomcool replied to tanner_shany24's topic in KX450FI realize I’m replying to an older thread here, but just in case, here’s my experience with this issue. I had a similar problem when I purchased a used 450. I rebuilt everything top to bottom and still had coolant/oil mixing on my first attempt. In short, the head was cracked and had to be replaced. You mentioned that yours had been very hot and was warped on inspection. I’d look very closely at the head just under the cam journals. Good luck.