I have been curious about how custom parts work for a while. For example, you can put in a custom part from the spec view palette, and it will be dynamic. But if you put a part from the base catalogs in the custom parts spec, it wouldn’t work.
While at AU 2014, I discovered a little trick. It turns out that what makes custom parts special is the Component Designation property. In the spec, all of the custom parts have Component Designation = Custom. You can set this property on any part in any spec and the program will allow it to be changed dynamically.
Here is what the inserted part looks like:
The only wrinkle I found is that while the graphics change and the valve gets longer, the property for Length does not. However, this would only affect the reports, as the isos show the correct dimensions.
My recommendation is to use this feature on control valves, relief valves, and other potentially changing parts, which will give you the flexibility of using default catalog items dynamically within any spec.