Adam,
A point to always bear in mind when working with SU are its limitations when modelling circles, cylinders, arcs etc.
Remember that circles, cylinders, pipework etc are made up of short straight line segments and are not really circular at all, but many sided polygons that "cut the corner" of the circles etc they are approximating.
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When importing CAD data eg from AutoCad, the original geometry may be modelled as solids etc and have no problems at all in their original (CAD) form with respect to gaps, slivers, fit etc.
When these solids are imported into SU, all the cylinders, pipes etc are approximated with many sided polygons.
This sometimes creates gaps and slivers where the original geometry in its CAD format had a perfect fit with no gaps at all.
Particularly noticeable if you zoom in.
On the whole SU does a pretty good job when importing such curved CAD data - but it can only ever approximate the original.
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Hope this helps
Howard L'