picked up an 08 Yz 450 that was seized up. owner said that the air filter had came off while riding in the desert.
Tore engine down completely found metallic pieces of metal in oil and mild seize marks on the piston skirt predominantly on the exhaust side. rings appeared ok as did the crown and ring lands. inspected the transmission gears all looked good. Crank big bearing axial side play was about .012-13 and rod spins freely on big end. Not much radial play "by feel". .002 is spec. Replaced the center intake valve and had the seat cut all of the remaining valves looked good for 142 hours on the meter. Was told the piston was replaced at 100 hours as a preventative measure. reassembled the bottom end installed a new cylinder and vertex piston from cylinder works. Bike ran great for .8 hours and I began to notice that upon letting off the throttle I could tell it was trying to seize again as I felt the rear tire lock up on a jump but fired right back under more throttle. Shut it off and drained oil and filter only to find the same metal in the filter and oil. Identical piston damage only this time the cylinder was not damaged. The oil pump gears measure good and all of the oil passages and jet are clear. The piston was matched for the bore size so I know that it is not a piston fit or ring gap issue.
I know 142 hours is a fair amount on a crank but it good compared to ones I have replaced before . My KTM crank went bad once and shed metal like this and the piston was damaged in the same manner however the rod was so loose it was obviously bad. I did notice that the crank had a decent amount of axial play in the case but unlike the KTM there is no way to control the thrust. The case bearing roll just like new and don't make any noise in fact compared to a new set they seem about the same. Can too much radial big end bearing play cause the piston to seize? If so what is it causing the skirt to rock harder that it would under full throttle acceleration?
I worked on a CRF 450 that had similar but upon tear down I could get the rod to lock up at times by pulling while I turned it so it was clearly bad. ideas. manual has no spec on crankcase to crank axial clearance. I have no problem replacing the crank, cranck case bearings and piston but I want to understand the piston seizure. yes it was full of oil and I checked to see that oil was flowing by checking the bolt on the back of the cylinder on the first start up.