Hi Bernard, thanks for posting the model files, that really helps to figure out what is going on.

It looks like the problem here is that your object is just very slightly asymmetrical itself, here is a zoom in on the control points in the front view:



If you have the curves that were originally used to create this object, if you could post those as well it could help me to try and solve the asymmetrical surface creation in the first place. Probably this was a closed loft and that needs a tuneup.

Anyway, if you turn on control points, go to the front view and select the center line, then use Transform / Align with the "Vert center" option and align all those points to the origin, then that tunes it up enough so that the problems in your Screenshot_2.jpg (view from the Left) go away. The symmetry of the object still isn't quite perfect with just that one tune-up though, so there are still a couple of asymmetries from the front view.

But I think those won't happen if you tune the object up to be closer to exactly symmetrical.

The one you show for Screenshot_1.jpg is a different problem though - for that one if you set it to generate n-gons, you can see that the n-gons are symmetrical. The asymmetry here comes when the n-gons are turned into triangles.

In this case it would be better to turn an n-gon like that into 2 quads instead of triangulating it. I'll see if I can tune things up to do this, which would then solve that particular situation.

- Michael