Desmodromic valve operating mechanism is where the camshaft controls both the opening and closing of the valve. One benefit of this is that when the motor is over-revved, the valves are still controlled, whereas when they are returned by springs the valves can "float" and hit the piston. Another is that the manufacturer can use wilder cam grinds for better performance.
Ducati and Mercedes are two major companies who have used the desmodromic system in racing engines. Ducati has the most experience of any manufacturer in the world at successfully applying desmodromic valve control to production machines.
Note that in the diagram, the springs are only used to close the valve the
last fraction, to aid starting.
Valve clearances are adjusted by shims. The rocker arms have to be removed
to insert the shims.
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