Eletric Motor Water Housing

thingiverse

This is an outer shell for and eletric motor I got. This motor was in need of a cooling system, so I decided to start modeling an water cooling shell around it. The problem is that the model looks great at the 3D modeling tool. But when you open the file in a slicer, the hole where the motor goes in is filled with material. I still don't know what I'm doing wrong, so I'm uploading it here, hoping someone could help me...