How to close the inside face of a gear I created this gear (from the Blender additional meshes add-on), and I would like to fill the gap to make the inside like the inside of a cylinder:



I tried selecting the edges and typing F but that filled the top and bottom faces:



I added a cylinder inside the gear and tried F again but no success here either:





Not sure how to do this...

Blend:
If the rings on the top and bottom have the same number of vertices, you can select the top and bottom edges, then press the space bar and type bridge edge loops: