Note that you will also need the No. 55 - Circlip pliers to replace this assembly.
When replacing you need to remove the Cap and the Needlebar pin. Use the long end of the 1.5mm allen key ( the largest allen key supplied with the machine ) and insert it through the hole in the adjustment screw to loosen. Use the Circlip pliers to remove the Circlip and push out the Connecting pin. Put some grease or oil on the new piston assembly and insert it into the frame, make sure that the Stay up spring is in place and engages with the Spring stop screw in the frame. Insert the Connecting pin and install the Circlip. The Needlebar pin should have a clearance of aprox 0.5mm from the backside of the flange to the frame.