It has linear guides that limit the cylinder's motion. It won't twist as long as the steel rod is in. Plus, I made an "alignment hole" to properly align in the first place. I have video of the assembly of the valve that could demonstrate this but it is big (~60 MB). Furthermore, it didn't twist during testing. The spring doesn't provide a rotational force; it can't see how it would twist unless someone actually grabbed it and twisted it.
The guides also limit vertical motion; the cylinder cannot be removed without the removal of the steel rod. I thought the design through.