I am repairing an oil leak on a 2009 yz450 that was bought as a left over model in fall of 2010. It started leaking from behind the sprocket. Simple fix, right? Yep, so I replace the oil seal, collar, and oring. It held for about one 20 minute practice session. The next time I started it up I noticed it was pouring out, not just dripping. I thought maybe I had made a mistake and replaced the seal and oring again. This time it leaked instantly with the new parts. I figured maybe there was something wrong on the inside. I removed the engine, split the cases, inspected everything. A few of the bearing were questionable so I replaced them all as well as every oil seal, o ring, gaskets and other little questionable parts. I assembled everythng and for more security I put a small amount of yamabond around the outter edge of the drive axle seal. It did not leak for 30 minutes of ride time. Then I noticed it was starting to seep under the sprocket again. Now I am getting frustrated. I inspected everything as best as I can from the outside. The shaft does not appear to be bent with running the bike. The oil is coming out between the collar and the seal lip, I was able to watch it slowly come out at about the 10 or 11 o clock area and run down and start to drip. I noticed the bearing covers about 1/3 of the oil hole in the shaft when installed. Could this "blockage" be causing the seal the not hold. The bearing was fully bottomed. I decided to split the cases again and look at everything again even more closely. I am not finding any definite causes. What could this possibly be? If it was some type of defect in the shaft or the case I would think the problem would have happend much sooner. If I had unlimited income, I would replace the cases and the shaft but that just is not logical. Any help is greatly appreciated. I have pictures but cannot figure out how to upload them. Thank You!