Sun Mar 14, 2010 2:14 pm
The valves in the photo of the doubles in question are a standard 1/2" NPT (tapered pipe thread). I am almost certain that no manufacturer other than Sportsways ever used the 1/2" straight thread with O-ring seal. Keep in mind that pipe thread stated size isn't a measurement of the outside of the threads, but is a "nominal" measurement of the inside of a standard 1/2" pipe, so is quite a bit larger than 1/2" on the OD.
The bushings on the cylinders appear to be either 1" or 1.25" pipe threads. If they are the appropriate material for high pressure, i.e. not hardware store pipe fittings, then they are fine to use... but most dive shops will not fill bushed cylinders.
I don't share most peoples' opinion of the 1/2" NPT threads. I just don't find them that difficult to deal with, since I have a pipe vise that firmly holds the cylinder while removing and installing the valves. It is not really a problem with the right tools and the right inspection light. I have heard lately of additional concerns about "wearing out" tapered pipe threads from too many cycles of removal and replacement, but I have not found this to be a problem if you just don't over-tighten the valves. Use plenty of teflon tape. I use 5 to 6 wraps on 1/2" NPT - not the often stated "2 wraps", and It is not necessary to tighten to extreme torque. I don't have a torque figure for you, but after years of experience just have a feel for it. Too much torque may indeed stretch the cylinder neck, and/or cause galling or distortion of the brass valve threads.
The larger NPT thread sizes do require more torque for sealing. If you are using bushings, do not remove or tighten them into place without an appropriate valve or plug snugly installed into the female thread of a bushing, or you risk crushing or distorting the female thread of the bushing!
No offense to my friends who dislike the NPT threads, as they are certainly more trouble to remove and replace annually for inspections... but I do have a number of cylinders that have survived this nicely for many years, and they are Vintage appropriate for the 1950's and 60's!
The older I get the better I was.