Hello all;

  I'm new to this forum and though I have been using Solidworks since 2005 I would place my skill level at "enthusiatic amateur".  I am moldeling a gear box housing from which I will be creating patterns for casting the part out of aluminum.  I need to model internal oil passages on the inner surface of the gear box housing which would be easy enough if the surface was a flat plane.  But of course it isn't.  The inner surface of the housing is a large oval which morphs into a smaller circular surface.  There is a pretty generous radius that joins the two surface features.  I want to create an oil passage that lies on the inner surface of the housing.  This oil passage will be .375" in diameter and have a .250" wall thickness.  Basically, I want to create a tunnel and attach it to the inside of the housings compound curve surface.  I am thinking that the way to do it is to extrude a profile along a centerline that is placed equidistant above the housing's inner surface.  How do I project a centerline off of a surface?  Or is there a better or easier way to do this?  

                                                                                                                                                                             ZACK