I created some thread profiles and saved them as blocks. Now, in theory, if I need to model threads, all I need to do is create the helix with the right diameter and pitch, drop a block into a 2D sketch at the start of the helix, and sweep-cut.

It worked exactly as intended (profile sweep cut) when doing an external thread. Then I tried to do an internal thread, and:

1. If I select the sketch and the helix, and then click on the sweep cut button, the sidebar shows the helix selected as the profile and no path selected. So I clear that, select the helix as the path, but when I try to select the profile sketch, it refuses, and shows me the a small tool-tip type window with different selection tools like region select, normal select, etc.

2. If I edit the sketch, explode the block, and repeat the steps to get to sweep cut everything works as intended.

I didn't have to explode the block when I did the external thread. I don't think this is an internal/external issue, and the sketches I turned into blocks are very similar (and obviously don't have geometry issues since, when exploded, it works).

Any ideas?