Pneumatic Coupler Air Connectors PC4-M10 4MM Straight Fitting For PTFE Bowden Tube 3D Printer push to connect fitting for 4mm OD tubing, NPT 1/8″ thread. This item is often used in 3d printers to connect Bowden tubing to the extruder and to the hot end. This fitting has a 4.3mm output hole, so the tubing can be fixed into the connector.