Side of Active and overlaping mesh

I’ve been searching for about the last 3 hrs and can’t find an answer. I have a model I would like to use Mirror Modifier on the X axis. I selected a vertex on the X axis and select “Side of Active” and it will select every thing on the right side except any overlapping vertices as well as not selecting some vertices that are part of the right half but overlap into the left side of the X axis. The model is too complicated to select or deselect the rest by hand. I would really appreciate any help.
Thanks.

Try to use Symmetrize.

Example with that mesh :
http://i.imgur.com/nrauDDL.jpg

I want to symmetrize it on the X axis (with the origin point being the center of that function, so place it carefully), so first i select it all.

Then i press W -> Symmetrizse (or click on Mesh -> Symmetrize in the bottom header)

http://i.imgur.com/8miLBkr.jpg

Now let’s say it was the other side i wanted to symmetrize, so without touching anything else, i look into the Operator panel (bottom of the toolshelf on the left of the 3D view) , or press F6 to make it popup.
Then at Direction, instead of the default “-X to +X” , i change it to “+X to -X”
http://i.imgur.com/8zqaz3Q.jpg

http://i.imgur.com/GqZba5p.jpg

Now you can select a vertice on the center of the symmetry line, click on Select -> Side of Active , F6 to change the direction if needed, CTRL+I to invert the selection then delete.
Then you can add your mirror modifier
http://i.imgur.com/3qLddKR.jpg

Thank you so much!!! I didn’t expect such a quick response. That’s exactly what I needed

I jumped the gun. After trying the above, I still have the same problem. Some of the mesh that is part of the right half of the model, overlapping beyond the X axis into the left half is being deleted. Still stuck

Upload your blend ( http://www.pasteall.org/blend/ if you can’t attach it to your post) and i’ll take a look to see if i notice something.

I’m at work but will do that this evening. Thanks.

Are you trying to use a modifier (mirror) on selected group of vertices only?? This is allowed but you have to define a VG first. When you say the model is too complicated to select or deselect by hand it’s a little confusing without seeing the mesh but usually you can box-select and delete vertices on one side of any axis quite easily from one of the axial ortho views…

The symmetrize function works best on whole meshes - it will duplicate selected vertices across an axis but will not delete unselected vertices - in addition if the selection is partial and crosses the axis of symmetry some verts will be deleted and some not in a predictable but difficult to describe concisely way.

If you don’t want some vertices it’s a good idea to just delete them before starting a symmetry operation!