PROGRESSIVE action gas lift pistons. They are designed to hold the door in ANY postion during the opening or closing process.Simple Snap-on 'Ball & Socket' fitting action. NB - THESE PISTONS WILL NOT REPLACE THE OLDER SALICE/STABILUS TYPE PISTONS THAT FITTED WITH SCREW BOLTS AT EACH END!!! The old type are no longer available from Salice.7 x Strengths: