Non Manifold object ? why hole is not print with 3D Printer? I don't undestand when why i print my gear (with central hole) the hole (only on the top) is fill and not print.

There's problem in my blend project ?
Is non manifold ?

I hope someone can help me !

Sorry for my english...
Some normals are flipped, so go in Edit mode, select all and recalculate the normals with ctrlN