You got me... Yeah the models weren't simple to make to say the least. The issues arise in how organic the shapes are, this makes it very difficult in a professional CAD program. So to get around that, I used Blender (I model more efficiently with it than I do in 3DS Max or Maya) to make the shapes. Of course Blender is an artist's tool and not a machinist's, so all of the scaling systems are simply crap and don't work properly. Ended up doing many parts by eye and scaling other bits to individual Blender unit squares. Admittedly a lot of this is down to how I'm running on my spare HDD currently, which also doesn't have any pro apps installed either. I'm just relieved everything fits to be honest.