Based on your description of a M25 x 1.0 thread on a 232 bar bottle, I'd hazard a guess that you've got the crossflow valve from the top of a diving cylinder lying about somewhere handy. Have a look at the way the valve spindle on that is sealed (and yes, that gland does see 232 bar - it's a balanced valve, with cylinder pressure acting on both ends of the plug when the valve is open. The seal is just an O Ring working on a chrome plated brass spindle.
The amount of friction you see depends a lot on the diameter of the spindle, so keep that as thin as you can, consistent with being able to sustain the amount of torque you need to transmit. Two things you might consider to allow you to keep the diameter down:
1. Use a stronger material than the brass used in the original cylinder valve
2. Consider copying another aspect of the standard crossflow valve design - use a short spindle as a screwdriver to operate a fatter shaft which is supported at both ends, entirely within the pressure boundary and which has the necessary stiffness to span the length of the cylinder without distorting.
Final thought: The combination of "never really worked with pressure", "Home Made" and HP Air is setting off all manner of alarm bells. For sure, there's only one way to get beyond "never really worked with.." but bear in mind that this stuff can kill you/your neighbours while attracting lots of unwanted legal attention. I've not long finished writing up a report on a very small cylinder that exploded at a much lower pressure (and the injuries that caused). The last I heard, the lawyers were still talking. At the very least, you will want to:
1. Make sure that your new top closure is designed to stay in shape and in place (with >10 kN underneath trying to change both of those things)
2. Same for whatever you've designed at the bottom end of the cylinder
3. If this is an adaptation of an old cylinder, understand whether you've modified the cylinder body in any way, and what that means for its strength. If you have changed anything (even the threads), you're going to want to hydrotest it. If the cylinder isn't in the first flush of youth, you're going to want to hydrotest it regardless.
4. If the cylinder body is home made, then if it has an internal volume of more than a few teaspoonsful, retreat to a safe distance now and wait.
5. If you haven't done so already, find out what the law in your part of the world says about the design of pressure containers. It may well mandate application of design standards, type testing and application of a written scheme of examination - all underwritten by (at least) a "competent person". In lots of places, these laws are enacted in ways that mean they don't just apply to the workplace.
6. Think about whether there's any risk of shearing your valve spindle. If there is, design it so it breaks in a place that won't turn the spindle into a projectile.
....and please share (if you can) what you're operating at the other end of the cylinder - you've got me curious now.
A.
