This is from Architecture, but should be the same as MEP. Read below from the Help file....
When you import a DWG or DXF file, each layer in the file is assigned a line weight based on the pen number-line weight settings. Revit Architecture can import pen numbers from a DWG or DXF file and map them to a Revit Architecture line weight. You can then save these mappings in a text file, and they become the set mappings for the project.
Revit Architecture includes the following files with pen and line weight mappings:
-importlineweights-dwg-AIA.txt
-importlineweights-dwg-BS1192.txt
-importlineweights-dwg-ISO13657.txt
-importlineweights-dwg-CP83.txt
These files reside in the Data folder of the Revit Architecture installation directory.
To set line weights
1. Click File menuImport/Export SettingsImport Line Weights DWG/DXF.
The Import Line Weights dialog displays the mappings in the importlineweights-dwg-default.txt file.
2. If this is not the file that you want to edit, click Load, navigate to the correct mappings file, and open it.
3. In the dialog, match the appropriate pen to the appropriate line weight (for example: Pen Number 1 to Line Weight Number 1, Pen Number 2 to Line Weight Number 2, and so on). Set as many pen-line weight mappings as desired.
4. Click OK, or to save the mappings in a new file, click Save As.