While using the partbuilder to modify/create custom pipes and structures I ran into a bit of a problem. I created new chapters in the US ImperialStructures Catalog to store my custom structures. When I did this I got multiple listings of the same structure(s) within the new chapter. Do not deletes these duplicates!!!!! Deleting any one of them deletes your new structure!!!
You can however fix it ......
If you've reached this point you realize you're dealing with an undocumented command and like living on the edge. Edit the "US Imperial Structures.apc" and remove the redundant information for your new structure. The repeated information is sequential and pretty easy to spot. Apparently this file doesn't get updated correctly.
Use partbuilder with extreme caution.