Hello guys, i was just passing by and would like to share my experience, i had a model which is contain more than 1500 mesh. I crafted 2 uv, first one for texture and the second one for light map in 3ds max. Whenever import to this giant structure (without compined mesh !) lightmap build perfectly working, but whenever combined this meshes during the import lightmap is not working ! what worked for me: i bring this structure without combining box checked and building the lightmap then merging meshes from developer tools>merge actors… But i needed this structure as one piece regarding to interaction. And also considering performance with LOD ! in that point after generating LOD, light map completely screwed up ! Now i gave up with light map, now my giant once piece mesh is movable and performance is perfect with LOD… For your info guys