I measured the force require to crack open the valves on a number of my 4 stroke engines. Most engines required a force of between 1lb to 2lb (approx 0.5 to 1.0 kg) applied to the end of the valve stem to start to open them. The only exception was my Seidel 5 cylinder radial which required a force of 4 lb (approx 2.0 kg) to start to open the valves.
It's was not that difficult make the measurement using a long push rod to apply the force to an upside down spring balance. File a V in the end the push rod where it pushes against the spring balance's hook, to stop it slipping off
Sorry about the shaky photo, pressing the camera button with my left hand
Mike
Mike