I'm trying to save an assembly out into a solid body part file. The original assembly is composed of solid body parts.

When I go to "save as" and choose .sldprt the file comes out with all of the bodies converted to surfaces. I can't even knit these bodies as there's a lot of missing surfaces.

So then I tried to save the same assembly as a .step file and once opened saved again as a .sldprt. I get the same surface body result.

Any idea what I'm doing wrong here?

The goal of my end result is to have a saved out cross-section cad-model of my assembly.