I know that I did something that probably was not according to the rules. I have been making an assembly of the individual parts as I progress with the overall design. I was not sure if what I was trying to change would work so I had a part "cylinder" so I saved as "cylinderMOD" and made the changes. Once I was sure that it would work I deleted the original and renamed CylinderMod to Cylinder and placed the modified part in the assembly. Everything was just fine until I closed and reloaded the assembly. Now it keeps telling me that Cylinder cannot be found. That is true. The original no longer exists. I did not want to lose the original until I was sure that the modification was going to work. I am not sure of how to correct this. How do I make it s that the assembly does not keep looking for a part which does not exist?