How tight does a valve need to be in order for it to burn?

I believe the valve burns because of inadequate heat transfer from the valve to the valve seat due to it being held open. Is this correct?

Are larger valves (ie two valve per cylinder versus four) more prone to burn at an insufficent clearance versus smaller valves?

Is it usually an exhaust valve that burns?

Any experiences or thoughts are appreciated.

Tinker

Any valve can burn! Lack of HEAT transfer is most common reason for failure. However a defect in the valve can cause the valve to burn as well.
MOST MB exhaust valves are sodium filled to help with heat transfer.

Since most engines have hydraulic lifters now, we see less burnt valves than on earlier engines.

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