and corresponding linkage as previously described it is possible to displace the rotation centre of lever 3 so as to reduce the clearance by a value u201c u25a1 u201d obviously a rotation in the opposite direction of a value u201c u2212 u25a1 u201d must be performed to eliminate the decompression braking effect of the gas within the cylinder thus returning the eccentricity u201c e u201d to the predetermined value if valve 5 is not actuated by the cam portion corresponding to compression braking such rotation may be performed also without the need for additional return devices indeed lacking the action of the actuator the force exerted on lever 3 during the actuation of the valve by the main cam portion is sufficient to return the bushing to its original position a better control of the linkage may be obtained by adopting a possible return spring not shown in the given example which facilitates the return of the eccentric bushings to the position they occupied before the actuation of the actuator lever axle 2 and shaft 6 are secured in any appropriate manner to the cylinder head shaft 6 being free to rotate as mentioned according to a particular embodiment of the invention described with the aid of fig5 a.