The next step in the design was to replace the mechanical push rod with a hydraulic actuator to open the valve which was closed again by using the mechanical spring. Later versions on the current models show that mechanical spring has been replaced with a compressed air return spring (figure 2.45). The cylinder cover is manufactured from forged steel and cooling is accomplished through radial cooling bores close to the combustion chamber surface. The exhaust valve cage and seat ring are also bore cooled.