Hello, I've tested this on 1.6.3 and 1.6.1, with the same results in each.

I have this part that is essentially a tube with threading on the inside, but when it gets sliced it appears to print a solid mass in the region where the threading is.

Are there any suggestions for this? The 3D model seems to be fine, it renders approprately in the slicer, it's just the slicing that seems to have this issue.