A piston assembly for a door closer includes a piston formed with a mounting hole and a door closer valve installed with clearance in the mounting hole. The door closer valve has a body part with first and second formations at opposite respective ends of the body part for engaging opposite respective sides of the piston and retaining the valve in the mounting hole. The body part is movable in the mounting hole axially of the mounting hole. The first formation is a protruding flange for seating against the piston at the first side thereof around the mounting hole and creating a seal around the mounting hole.