Originally Posted by Left C
CGA-320 reads are not like NPT tapered threads that need pipe dope or Teflon tape for sealing. A washer fits between the male and female ends that forms a leak free seal when tightened properly.
"Tape is neither necessary nor especially useful. It won't improve the seal and you shouldn't expect it to. The tank should have CGA 320 fitting on it with male threads and the regulator should have CGA 320 female fitting. CGA 320 fittings are *not* like standard pipe threads. With standard pipe threads, the tapered threads provide the seal and using tape or pipe dough helps ensure that the threads don't have any small gaps. But the CGA fittings *do not* seal at the threads! They seal where the flat faces of the two fittings meet and that's why you *must* have a fiber or nylon washer between the two -- to seal the joint where those to seats meet. In fact, applying tape makes it possible to bind the threads before the fittings are fully seated against the washer -- although it would take a lot of tape to do that. ..."
"The valve used in the cylinder is typically made out of forged brass and in the US has a Compressed Gas Association (CGA) connection type 320 (see diagram) that requires a flat washer to provide the seal. This washer should be replaced every time the regulator is reconnected. Do not use Teflon tape or any other type of sealant but the washer. The company filling the cylinder can provide you with new washers. Note that there are different valve and connection specifications throughout the world, so insure you check your local specifications."
I think this is the best description with pictures as well. On a compression type fitting the tape does nothing to help assure a seal. The seal is the washer or gasket top of the bottle and the flange around the regulator. If the gasket or o ring were to leak the tape still would not help as the leak would be around the top hole in the compression nut rather than at its threads.
Now if you had a pipe threading type connection then taping can be a life savior especially if through usage the threads got nicked.