A grain bin located atop a harvester includes upstanding walls having a substantially continuous upper edge defining an opening. A grain bin extension is deployable between extended and retracted positions to expand the capacity of the grain bin. The extension includes extension panels and a cap overlaying the opening. The cap is supported by a cap frame that is also deployable between extended and retracted positions. An extension deployment apparatus raises and lowers the extension panels. Extension deployment mechanisms located at opposing corners of the grain bin control the deployment of adjacent extension panels. Each extension deployment mechanism has a first rack and a trunnion to drive extension panels in a vertical direction and pivot the panels in an outward direction and a second rack mounted to the trunnion to drive the cap frame in a vertical direction, and a drive gear between the first and second racks.