Hi Billabong, I also found that if I hack off the back part of the object then it booleans ok too:



I noticed that the main object is sometimes not being categorized as a solid even though there are no naked edges. This means that some piece of the model is not responding well to ray fire tests that are used for volume containment calculations.

I've been going around and removing chunks of the model to try and weed out the problem geometry and I think these may be an issue:





It's easy for rounded corners like that baked directly into the surface control points can end up as a self intersecting surface.

Some info here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1127.4
http://moi3d.com/forum/messages.php?webtag=MOI&msg=3473.3

- Michael