So, went to time the cams in the boys 2007 YZ450F, he had adjusted the valves and timed the cams.. After the test ride he heard a "noise" and the bike stopped running. I got home, we re-timed the cams..... and at just before TDC Compression stroke, found the valves to be hitting the piston.... Figured he had bent a valve with mis timed cams.. Checked valve clearance.. No new open gaps Looked at valve cotters,, none popped up or out Both signs of crashed / bent valves. Re timed cams, triple check cam, crank timing.. Something still wrong,, intake valves hitting piston.... Checked nut on flywheel,, it was a bit loose, not hand tight, but not much more.. woodruff key was still present. Checked timing set up one more time.. all was well.. So off came the head...... NO BENT VALVES But ..what then... only thing left was crank to flywheel index. Pulled the cover, pulled the GYTR light weight flywheel...... and yup.. it was spun on the crank 10deg or so. Key was not sheared. best guess is..... My son did not tq the flywheel nut sufficiently when he installed the flywheel,, nut backed off... flywheel broke taper.. spun... effectively making the crank / cam timing off even with the marks properly aligned. In hind sight.. I could have diagnosed this by installing a timing wheel and piston stop.. verifying the flywheel timing marks. or I could have used a spark plug dial indicator set up I have to verify flywheel timing marks. I was just more focused on my son telling me he had mistimed the intake cam 1 tooth... and now we had the valve inference issue.... In hindsight.. I now know the cam was mistimed due to the crank issue... he likely installed the cams perfectly It will still need valves soon (tightened twice in 19 hours) at $344 parts cost alone.. Not super happy about that... though Yamaha's did not wear though valves this quick (I'm the third owner.. no way to know what kind of airfilter maint was done by PO's, RPM use, etc etc) The better news is.... Cody did it right when he adjusted the valves and set timing.. He missed on the flywheel nut tq some weeks back,,, but, well we all learned from our successes and mistakes,, he is no different.