Robart Air Line Retainer Fitting (6) This is a package of Air Line Retainer Fittings from Robart. FEATURES Knurled for tool free hand tightening. Aluminum construction. Threaded through entire body. INCLUDES Six air line retainer fittings. REQUIREMENTS Assembly on air tubuing and retract fittings. SPECIFICATIONS Height: 6.0mm (.238") Outside diameter: 6.4mm (.252") Inside diameter: 4.0mm (.159") COMMENTS Air line fitting retainers connect your air tubing to your fittings. The tubing goes to the fitting, and the retainter holds it in place by threading over the tubing and the fitting.