Originally Posted by onehundred80
That's what I like to see, someone using a little initiative, take the OEM part which is well made and better than the C3 and adapt it. Well done. 
The groove is probably the division point between two different diameters, one a bearing fit and the other a shaft fit, with metric standard sizes and tolerances for both.
Edit
Just seen the last post, do not weld it as I am sure you know. That could ruin it really well.
Precision parts are not welded unless every think is welded and totally re machined. The welding would be done with a laser and very carefully, no bloody great spark and heat.