There is a missing part at my object when I render in cycles This is my model in the solid view, we can see it is complete.



but in the render view or in the final render it is clipped :





here is my viewport clipping settings :



I have added a cube to the object in edit mode ; the render shows me all the form but not the cube!!!! as you can see 



to clarify:

I added to the object (in edit form) a cube and move it far away. Now the render shows all the form as if the new mesh is moving some clipping plane...
I don't entirely understand what you were doing with the troubleshooting, but the problem may be the camera clipping.

In the Properties Panel (N while cursor is in 3D view) you can change the viewport clipping.
By selecting the camera and going to the camera properties you can change the camera clipping.



Increasing these values will allow more of the scene to be rendered.