Hello Unrel People! 
I try to cut hole in sphere with modeling tool>TriModel>DeleteTriangles
But its works only for outside:
Nothing happens, if look from other side. 
Plase, help me to understand, how to fix this.
THANK YOU UNREAL PEOPLE. 
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That mesh is two sided, you can tell this looking at this picture
You need to delete triangles on the inside too because they are duplicated (otherwise it would be invisible from the inside)
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Thanks a lot man, I try to find the button to do this. Now I got it.
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