You're right - its better to make the map produce no displacement, if what is desired is no displacement. If you have no intention of displacing a polygon, it doesn't matter if its welded or not.

On the other hand, if you do want to displace a cylinder, to make a rib or whatever, you have to have all the polygons welded.

We still have no idea if that is the issue with this locomotive.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)