Hi Nick, unfortunately the STEP file format has a kind of different way of representing trim curves - the way the file structure is set up trim curves are only present as 3D curves and they do not necessarily map one-to-one directly to UV parameter space curves.

So in order to convert those kinds of trims into the more conventional trimming boundaries that MoI uses (where each trim curve has both a 3D edge and a UV parameter space edge to match it), those 3D curves need to be processed and one of the tricky parts is that 3D curves that cross over a closed surface boundary (like the seam edge of a cylinder for example) need to be cut at that crossing point.

That is a somewhat delicate operation and can be hard to resolve in cases where trim curves are kind of barely skimming along the seam.

It tends to be in that particular area where STEP imports can potentially get messed up when importing into MoI. If you notice those messed up pieces all have underlying surfaces that are closed surfaces with seam edges on them like a cylinder.

When that happens, it is can be possible to repair the file by doing some manual untrimming and retrimming on the pieces that have mangled trim boundaries.

There is a tutorial on how to do untrimming and retrimming here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

But probably the easiest thing to try in this case is to see what happens using IGES or SAT format instead, those formats do not have quite the same kind of trimming issues as STEP format, it is possible for there to be some other kinds of issues instead, but hopefully one of those other formats will handle this particular shape better.

Let me know if neither of those formats helps get it into MoI, and I will try to help you get the file in using some other program as an intermediate step, or I can help do the repair work on it if that's the only option.

- Michael