Our pressure vessels are mounted vertically on a cylindrical skirt (a very common method).  Past designers (including me) have just fudged the mate by specifying an estimated distance.

I would like to actually use a proper mate of the surface of the welded end cap resting on the inner diameter of the cylindrical skirt.  One issue right away is that I can't seem to select the ellipsoid portion of the weld cap as a surface to mate to.  For accuracy, this mate is being done within a multi-body part, not an assembly.

I'll see if I can get a picture for clarification added.  Thanks!

Jim