A method to control the size of a combustion chamber by an actuator in the cylinder head of a piston combustion engine, said piston combustion engine comprising: - a combustion chamber; - an engine piston mounted to a crank shaft; wherein at least a substantial part of the combustion chamber is present above the engine piston in the cylinder head; - the actuator comprising: a vertically displaceable piston arranged in the combustion chamber for providing variable compression ratio, a first chamber with a first shaft extending through the first chamber and being associated with the vertically displaceable piston, the first shaft having a flange and a spring disposed in the first chamber, the spring acting between the flange and the first chamber floor to force the vertically displaceable piston in the upward direction, a second chamber and a third chamber filled with hydraulic fluid and separated by a valve with an opening, wherein the valve being horizontally repositionable via an electromagnet, a step motor and a second shaft vertically displaceable by said step motor in the second chamber, wherein an upper part of the first shaft extends into the third chamber such that the hydraulic fluid may act thereon, the method comprising changing the size of the combustion chamber by: displacing the valve with the electromagnet so that its opening connects the second and third chambers (14, 17), and displacing the second shaft using the step motor to cause a flow of hydraulic fluid between the second chamber and the third chamber, thereby repositioning the first shaft and the associated vertically displaceable piston. - a combustion chamber; - an engine piston mounted to a crank shaft; wherein at least a substantial part of the combustion chamber is present above the engine piston in the cylinder head; - the actuator comprising: a vertically displaceable piston arranged in the combustion chamber for providing variable compression ratio, a first chamber with a first shaft extending through the first chamber and being associated with the vertically displaceable piston, the first shaft having a flange and a spring disposed in the first chamber, the spring acting between the flange and the first chamber floor to force the vertically displaceable piston in the upward direction, a second chamber and a third chamber filled with hydraulic fluid and separated by a valve with an opening, wherein the valve being horizontally repositionable via an electromagnet, a step motor and a second shaft vertically displaceable by said step motor in the second chamber, wherein an upper part of the first shaft extends into the third chamber such that the hydraulic fluid may act thereon, a vertically displaceable piston arranged in the combustion chamber for providing variable compression ratio, a first chamber with a first shaft extending through the first chamber and being associated with the vertically displaceable piston, the first shaft having a flange and a spring disposed in the first chamber, the spring acting between the flange and the first chamber floor to force the vertically displaceable piston in the upward direction, a second chamber and a third chamber filled with hydraulic fluid and separated by a valve with an opening, wherein the valve being horizontally repositionable via an electromagnet, a step motor and a second shaft vertically displaceable by said step motor in the second chamber, wherein an upper part of the first shaft extends into the third chamber such that the hydraulic fluid may act thereon, displacing the valve with the electromagnet so that its opening connects the second and third chambers (14, 17), and displacing the second shaft using the step motor to cause a flow of hydraulic fluid between the second chamber and the third chamber, thereby repositioning the first shaft and the associated vertically displaceable piston.