This is a Robart Robostrut Ready 90?ø Main Gear Set with Air Kit. FEATURES: The 510 Series Retract offers a high performance air up and spring down. Setup from the factory to accept 3/8" Robostruts (ROBQ1700) Main Gear Set designed for the 510 Series Retracts The 510 Series Economy Retracts are designed to replace other manufacturer"s Retract systems. INCLUDES: 510RS Main Retracts Air kit: -Small air tank -Control valve -Fill valve -Air supply in fitting -Three tee fittings -Orange air line Instruction sheet REQUIRES: Robostruts (ROBQ1700) Air pump (ROBQ2363) Link rod assembly to the Control Valve SPECS: Main Retracts: Length: 3-5/8" (including air cylinders) Width: 1-3/4" Depth: 1-1/8" Robostrut mounting size: 3/8" RobartPart ROB510RS