I'd like to find a short story from one of the SF magazines — possibly Astounding/Analog— probably before 1965. I saw it a few years ago while flicking through second-hand magazines in a bookshop, and don't remember many details. However, I am certain that it involved a region of space with artificially altered properties. This was almost certainly inside a spaceship, and probably part of the propulsion system. The bit I am most certain of is that there were two "demarcation lines" between this altered space and the normal space of the ship's interior. "Demarcation line" was the actual phrase used. I am also pretty sure of the spatial distortion a crew member undergoes when inside the altered space, see next paragraph. Those are two key points that I remember. The third is a sense of threat or foreboding about passing a demarcation line.
I cannot remember where the ship was going, the names of the people on it, whether it was a research vessel, etc. I am reasonably sure that it wasn't a warship and wasn't in a war. What I do remember is that in order to solve a technical problem with the system, one of the crew has to enter the altered space by passing the first demarcation line, and then get through to the demarcation line at the other end. His experiences in the altered space are probably the main theme of the story. Once inside it, he deforms. Not so much that he dies, but I seem to recall him feeling as though his body has stretched to an enormous length. Despite this, he solves the problem and reaches the exit. It's possible that the space was inside some kind of tube or cylinder: I have a vague image of this being about the distance between one floor of a building and the next, and perhaps periscope-shaped.
This was a short story, probably less than 10 pages. I don't recall any illustrations, but may just have forgotten it or them. I've dated the story between 1950-1965 on the grounds that I usually remember if a magazine is pre-1950, and that I think the magazine was most likely Astounding/Analog but the cover wasn't a post-1965 style. I have never seen the story anywhere else, and attempts to trace it by Googling for likely phrases have failed.
Tha author definitely wasn't Asimov, Clarke or Heinlein. The story probably didn't have the level of scientific rigour that I've encountered in e.g. Asimov's "Billiard Ball" or Clarke's "Superiority". On the other hand, there was some attempt to justify the physics. Although the concept sounds like a wormhole, I don't think the words "wormhole" (or "Einstein-Rosen bridge", "black hole", "singularity" etc.) were used. Moreover, the spatial distortion, though daunting, was not enough to kill the person experiencing it, and I don't remember any mention of extreme quantities such as billions of gravities, body-rending tidal forces, or entry points only a few nanometers in diameter.
The magazine was in a bookbox that included a copy of Science Fantasy 48 (August 1961). I identified that by remembering that it contained John Brunner's short story "The Analysts", which I then confirmed via the covers shown at The Science Fiction, Fantasy, & Weird Fiction Magazine Index. So it's possible that my story was from around the same time.
This is consistent with me thinking that it was in an Analog with the title in all lower case. From the covers at Luminist's Analog covers page, that would set it between December 1961 and March 1965.
Having said that, I could be wrong about the period and the magazine. Judging by P. F. Woods's (Barrington Bayley's) "The Radius Riders" and Colin Kapp's "Lambda 1", the concept and style would not be out of place in, for example, Science Fiction Adventures or John Carnell's New Worlds. And as far as period goes, Wikipedia's Wormholes in fiction tells me that Jack Williamson used the idea of spatial distortion as early as 1931.