You may have noticed that some parts retain certain measurments in Imperial units even if they are a Metric part in a Metric catalog. For example the Minimum Curve Radius will display as inches even with a Metric part. These are even "greyed out" in the part properties which makes it even more confusing.
This is because these units are being controlled by the AeccPartParamCfg.xml file that is typically located here:
C:\ProgramData\Autodesk\C3D 2013\enu\Pipes Catalog\Aecc Shared Content
Look for the line calling out the parameter you want to edit and change the units. Example:
<AeccDfParameter name="MCR" desc="Minimum Curve Radius" context="MinCurveRadius" index="0" datatype="Float" usage="Double_Distance" unit="in" visible="True" internal="True"/>
Once edited this should populate within Civil 3D. Please note that this a "shared" file and is shared between the Imperial and Metric folders so you will need to change it back if need be.
Hope this helps!