I'm new here, but I have used the site many times for help. I have a 2006 YZ450F that was purchased used. When I purchased it the bottom end had just been re-built with one ride on it. I rode it for a few months and on my last ride it started smoking out the over flow tube from the top end. It was a white smoke. I rode back to the car as I felt my clutch tighten up. I put it in the truck and took it home when I changed the oil it was black(only one ride on fresh oil). I then took a look at the radiator and it was empty! So I started to fill it up and it leaked from the tube that goes into the head and I found that the o-ring was bad. So thinking I had found the problem I replaced the o-ring, changed the oil, and everything felt fine. I rode it again and the radiator fliud was gushing out the radiator overflow tube now at high RPMs. I assumed it was a head gasket. So I took it down to the head gasket replaced it, along with my clutch that I found out was about to go(jet black with grooves). When I built it back up it wouldn't start. I then thought I over tightened my cams and that was true(I recomened 5 foot lbs not 7) I losened them and it would kick over but not start. My mecanic has a compression tester and we found that I was losing 40% compression, thats the piston or rings. After taking it back down to the head I found that the pison has wear in it and the cylinder was also worn down and the oil channels were almost gone and on top of that my oil pump was about to go....what a drama! So now I'm looking to replace my piston, cylinder, oil pump, clutch, etc...has anyone ever had a problem like this, any word of advice as I proceed on the fix? Anything else I should look at?