Thanks, Michael. I did notice the gap, so I straightened the surface points along the join. Even without the gap it would not shell in Moi. It did in Rhino though, so maybe the tolerances in Rhino are looser for this option, although the general tolerance I had set to .001, the same as MoI.

I guess trimming larger surfaces is the way to go. I still think in a Solidworks way. In SW to create this surface I would have created helper planes along the edges then used a patch with surface continuity to the helper planes. Is there a similar method I can use in MoI as I am still struggling a bit with MoI and complex surfaces? I created this surface with a sweep, with maintain tangent set, which probably accounts for severely curved edge, which is causing the problem.

This canopy is particularly difficult as the aircraft windshield and side widows are flat. The frame where the sliding canopy joins it is basically flat sided. On this model of the Tempest where they introduced the bubble canopy, the bubble part has to transition to basically straight edges at the front, so it closes with the windscreen frame. I guess that they made the mold with wood and just sandpapered it until it fitted!