Variable length piston assemblies for engine valve actuation systems

In an internal combustion engine comprising a valve train for actuating one or more engine valves, a variable-length assembly for controlling motion applied to one or more of the engine valves comprising:

• a housing;
• a piston disposed within the housing and having a piston working surface defined thereon, the piston working surface including at least one piston working surface ramped portion;
• an actuating plate cooperatively associated with the piston and having an actuating plate working surface defined thereon, the actuating plate working surface including at least one actuating plate working surface ramped portion for selectively contacting the at least one piston working surface ramped portion;
• a retaining assembly for retaining at least one of the actuating plate or the piston in the housing;
• an actuating assembly for rotating one of the piston or the actuating plate relative to the housing wherein rotation of the piston or actuating plate causes the variable-length assembly to change length, from a minimum length to a maximum length, to take up clearance in the valvetrain, and wherein the piston working surface and the actuating plate working surface are arranged to remain in contact with one another throughout the change in length of the variable-length assembly from the minimum length to the maximum length.