I'm with Luis. The tapered threads put increasing outward pressure on the inside thread diameter the further they are screwed in. While there are many modern aluminum pressurized devices that utilize the NPT (national pipe taper) threads, they are likely of a different alloy. I do not know the alloy of those old Navy cylinders, but knowing that many other aluminum cylinders in the 1970's and through 88-89 were made with an alloy that proved to be susceptible to neck cracks, the Navy ones are highly suspect in my opinion. The later aluminum cylinders used straight threads with oring sealing, which did not tend to substantially stress the neck, but many of them still cracked at that area anyway.
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