Hi Sneather..welcome aboard :

when you open the export menu' in Moi try to control your model just in shaded mode (without showing polygons) this is how will render later in your application

I opened your model and try to export it,you can see a surface not joined properly,maybe that surface was modeled after main cylinder(see the mesh completely different of the rest)






so i deleted that patch and rebuild a new one by sweep:
blue=section
green rails
using your arc to cut the new patch (white)
this is a quick method just to show you the problem,sweep don't give a continuity with the main surface



then joined new patch with the main surface:no more cracks



you have other surfaces not correctely joined...



so i exported in C4D:you can see same problems



my suggestion is: ask to your customer a file in STEP format if it's possible and before exporting have a look in shaded moded,rotate your model,have a zoom to see closely...it not depends by Moi at all !