Please elaborate a bit more about what fittings you are referring to. And the background for your question.

J C... such as typical part numbers for the 'beam seals' you have in mind...

Regards, Wil Taylor
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I used a beam seal concept making a pressure tank of large diameter tubing. machined end caps with a beam feature that sealed it without the use of an o ring cause nothing was compatible for any reasonable amount of time. Even the fanciest elastomers on the planet would not reliably last 100 hours before failing. Lets just say the pressure was... HIGH enough there wasn't a phase diagram to specify what state the matter was in.

Why someone would use a beam seal on typical hydraulic fittings... no idea. I've never done it knowingly. Just looked up MIL-F-85421 see if any of the cited references lead to anything.

MIL-F-85421 is now AS-85421. According to manufacturer's websites, beam seal fittings perform better under high vibration, temperature and pressure conditions...better than what is left open.

https://www.mfcp.com/hubfs/MFCP/Catalogs/Stratofle...

Thanks for responses. I don't know any specific part numbers. My goal is to make assessment does it make sense to use such fittings et al. But, if they can last only 100 hours, then it is defintelly not for general aviation where my apllication is used.

"But, if they can last only 100 hours, "

That's not what RoarkS said, he was speaking of fittings that used elastomer seals. Life of a beam-seal fitting, like any other all-metal seal, is potentially infinite.