misterroy wrote:
Thanks Bob, I hadn't thought to taper it to get a seal.

Tapered fittings usually have the male & female tapered to match. E.g.,
the US NPT. Tapering just one would mean that only a couple of threads,
at the most, would be fulled seated. You might get a good-enough seal,
especially since you only need it long enough to get the cylinder to
pressure. Or maybe not.

Made-in-brass might work better - it would deform more easily than
steel, to match the existing threads.

Bob