Hi Burr,

Yeah, with NURBS accuracy is important and there are 3 words I will use here, Zoom, Zoom, Zoom. ;)

This is what I found that might be causing the problem, in the screen dump your eyelet on the sinker is not exactly in line with the body, it's about 0.00124 units out.
So when you try to split it exactly in half it try's to split those surfaces around that misaligned area into tiny surfaces which it can't handle and fails.
Michael will probably elaborate on this but I think that is the problem try and get everything aligned nicely first and give it another go.