Scaling the bore diameter
![](https://singedpinions.com/wp-content/uploads/2020/04/gear24.png?w=1024)
Start by merging via edge-sliding the extra edge-loops surrounding the inner surfaces. With x-ray mode on and in side view, circle select the inner circle.
![](https://singedpinions.com/wp-content/uploads/2020/04/gear26.png?w=1024)
Scale it in the Y and Z axis by the new bore diameter divided by the original bore diameter.
![](https://singedpinions.com/wp-content/uploads/2020/04/gear27.png?w=1024)
We are left with a bore diameter of 4mm.
![](https://singedpinions.com/wp-content/uploads/2020/04/gear28.png?w=1024)