I'm investigating various small LPG fittings, as used on things like blow lamps (and camping stoves etc) 'powered' from disposable gas canisters.
As far as I can tell, the thread used on most (all?) small 'tin' canisters with a threaded top is a 7/16 UNSF thread, with 28 TPI. (Taps for this thread are difficult to find or expensive, die are as rare as rocking horse poo.)
I assume the fine thread is used to secure a gas tight seal, perhaps through the use of the harder fitting and the softer canister threads. After all, the latter has a limited (planned) service life.
However, reading up on other fittings (BSP threads), which are widely used the thread pitch is far more coarse. Plus, the fittings tend to have a similar hardness. While various compounds etc. can be used to 'seal' the threads, I'm not convinced that accounts for the difference- especially if UNSF threads are used else where in gas systems ie besides canister connection.
(I appreciate that in compression fittings, it is the 'olive' which seals but I am not referring to those. )
Can any one shed some light on the topic, please?