Thanks Mike,
Very strange...
I just set up X3D to implement your suggestions and the first set of rivets went down to 'extrusion' depth 1. This is what I wanted from the beginning but this is the first time it has done what I set it to do. Now the other side won't do what it's told. I closed X3D and tried again to no avail.

Please don't sit back, even if I already know something, there will always be someone who doesn't.

Anas