Sorry guys I am stuck again on my motor mount.

I have a solid object - at least in the details box it says it is a solid.

However I want to shape one end of it to match the shape of the motor mount.

If I trim it using a curve, and delete the piece I don't want - the remaining part suddenly becomes hollow and reports itself as a set of open joined surfaces.

I have tried a simple line as well as the closed curve that outlines the motor plate. Both do the same thing. I have tried extruding the motor plate shape into a surface and then using trim and Boolean intersect. Same result.

I have read the documentation which shows both 3D and 2D intersections with solids producing a "carved" solid. It did work at the top end of the part. It doesn't at the bottom.

There is something about my solid that I don't understand. Can you help my understanding please. I attach a simplified model with the curve and straight line.

Thanks in advance