I recently bought a 2002 yz250f that needed the top end rebuilt but it came with all the parts needed to rebuild the top end, brand new wiseco piston and rings, the cylinder is either a 2002 or 2003 yz250f cylinder(used but in good condition), and the head is also either a 2002 or 2003 and a new timing chain. I owned this bike before back in 2014 i got it for cheap because it dropped a valve but i traded it because i couldn't get the money to fix it. Anyway i bought it back. My buddy and i put it back together. The newer(used) head needed a valve so we took a good valve from the old head to use in it. The bike won't start and while trying to start it the bike lost some compression and oil leaked out from the brand new lower head gasket. This bike hasn't ran since 2014 when it dropped a valve. It does drip oil but not from the new gaskets and it dripped oil before the initial compression loss. I dropped all my money into this bike($1100) and i only bought it for $600. Hopefully this weekend my buddy gets time so we can spend all day trying to figure out this bike and we have already checked the timing but we are not to good with timing so any help with all of this would be greatly appreciated. Also when my buddy and i work on it next we are going to tear down the top end again to fix the new gasket and to check and see if the piston rings set correctly. Again any help would be greatly appreciated this bike is a pain in the ass but i love this bike and as soon as it runs good i'm trading it for a 2 stroke 250. Also if this is posted in the wrong section sorry my bad this is only my second ever post on here and my first one was before i bought this bike.