Mirror Geometry Problem

I am trying to mirror the geometry of an early model of a head. When I select the head and mirror the geometry the open spaces on the head seem to fill instead of staying open (see imgs below).

polygons>mesh>mirror geometry





THE SOLUTION:
Check your merge threshold and lower you merge threshold if your mirrored object seems to be filling holes it should not be (see imgs below).

Above: Merger threshold set to 0......
NOTE: with 0 when you zoom up close there is a small separation between the two halves. This is best seen at where the bottom of the neck meets in the img above.

.01 works best with this model (see img below). Look at the neck and see how it is reacting to a .01 threshold. play with the threshold to make it perfect for your own objects.


Below: Merge threshold set to .1