With this I meant what Andrew said that the z edit field shows the wrong value. It should show this offset value if a drawing plane is defined before. Actually, I don't care whether the widget is disabled or not.Only the issue with wrong z coordinate should be fixed because it's a little bit annoying.
The problem I had was when drawing the initial spline. Since I had everything visible the snap function didn't always find the vertex but for my second try when hiding all irrelevant objects everything was fine.Actually it's an interesting but difficult problem... You want to move a point of a spline or wire. I can easily make inventor not see the control point (turn off the selectionMode field of the SoFCSelection node), but when you move the control point, the underlying vertex of the spline/wire gets relocated under the mouse cursor, resulting in being catched by snap... But you want to be able to snap to the spline itself, so you can close it... But doing so prevents snapping to other objects... It's a hard-to-solve philosophical issue
BTW, I think the dimensions and positions of the boxes under "Finishing out the Crankcase" are wrong -- in case I didn't make an error in my model.
The model I created I added to Subversion in the data/examples folder.