Thanks Grendel
but Thomas... I don't use the manifold tool at all !
the steps are :
1) create a curve
2) do a symetry of this curve
3) weld them
4) do a new symetry
5) do a ruled surface between the two curve
6) close
7) create a circle
8) extrude it
9) do a boolean
10) and to finish a little chamfer
but I agree with you... it should be better if I had done a manifold instead of the steps 1 to 6. It's an error from my part...