Springy variable displacement piston pump

The rotor has magnets spaced around it, the same pole facing towards the outside. As the rotor magnets move past a piston, they repel the piston due to its embedded magnet.

The distance between the piston and the rotor can be adjusted by supporting the piston with some adjustable- location magnets, which repel some piston-embedded magnets added for this purpose.

Changing this distance adjusts the stroke length of the pump.

For use as a pump you can use way valves.

If you use cams to control the valves you can run it as a hydraulic motor or as a pump.