I noticed you said you created this in rhino. Bring the mesh back into there and check to make sure the edges fo each group match exactly both in point count and in location. If they do not match exactly the mech will not get welded and you will get breaks. If the points do not match up you can fix it easily by turning point on (pton) and selecting a point on the chest and setting on snap to point and snapping it to it's corresponding point on the abdomen. If the number of points don't match then you will have more work to do by deleting points and recreating a few polygons so the edges of each group will match exactly.