Variable-length assemblies, including lost motion assemblies eliminate hydraulic or pneumatic working fluids for operation in internal combustion engine valve trains and may be integrated into valve rocker arm pivots. An example piston and actuating plate are provided with working surfaces that interact when the actuating plate is rotated relative to the piston. The working surfaces include ramped transition portions and may include upper and lower flat portions. A shallow ramp angle prevents counter-rotation of the actuating plate under load. Actuating assemblies include an actuating solenoid includes a plunger that engages and pivots the actuating arm to cause rotation of the actuating plate relative to the piston and changes the state of the lost motion assembly from an “off” state, where motion may be absorbed, to an “on” state where the lost motion assembly is rigid and does not absorb motion.