You cannot just install a valve rotator on any given valve. On some engines they sit above the spring and work also as a retainer, i.e. Chevy, Ford, Chrysler and many others. Some later model Chevy's had them on both intake and exhaust, it didn't solve the intake guide wear problem in these engines. On the Mercedes' that I have built the rotators are under the spring. There is a pocket recessed in the head to allow for the extra spring tension caused by the rotator. If you put a rotator under an intake spring you will cause excessive pressure on the spring and may even bind up the spring when the valve is opened fully, causing the retainer to destroy the stem seal.

The main causes of burned valves are; improper adjustment, excessive valve guide wear, pre-ignition/detonation, lean fuel mixture. When either of the first 2 conditions exist you have a constant compression loss in that cyl. When the piston comes up and fires the fuel, the flame is compressed through the gap and causes a torch like situation....hence burned valve. Pre-ignition/detonation occurs when the piston is on the way up and the fuel is fired (pre-ignited) which usually results in holes in pistons, burned intake valves and over-all DETONATION of the motor.