YZ450F 2010 - Valve problem, what could possibly be the cause

My friend's YZ450F has 63hours running, piston changed at 50hours. Suddenly, the bike cannot iddle. We open it up and found out that the Exh valve's top was grinding and the cotter was out of its place. Other valves' clearance are still in specs.

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Quick solution is grinding the top evenly and insert a thicker shim to ensure the standard clearance while waiting for new valve. The bike runs normal now.

We are still wondering what could possibly be the cause of this? Does anyone have the same problem?

I've seen this reported before. It would appear that there is a hard cap friction welded to the top of the valve stems, and that this has broken off that valve. You might see if it's laying in the head somewhere and be rid of it. If it were to drop between the crank and the cam chain it could be a disaster of spectacular proportion.

The keepers are out of place because of all the extra clearance at that valve battering the thing as it ran.

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