Member - 'Lucy' replied:

Primarily so you can't accidentally connect the gas bottle to anything else, and cause a 'fubar cloud' as Truckster would so eloquently put it.

This also fits the dangerous category.

LPG ( whatever variety) is for LPG thingo's only.

Member - Sand Man (SA) replied:

Only some do Alan.

To complicate things even further, different brands (primus, companion, etc.) not only have a left, or right hand thread, but also male, or female fittings.
(e.g. the thread is on the inside, rather than the outside of the fitting)

Although one can buy adapters to convert from one type to the other, it does make life more complicated.

I think its a conspiracy by the different manufacturers, who "force" their format onto us as a "standard" fitting. Problem is, it's only a standard companion, or Primus fitting, rather than a universal one.

Even further, some gas bottles use POL fittings. (like the common backyard barbecue, for instance) instead of the 3/8" fitting, more common on smaller size gas bottles.

The POL fitting is only of one type and yes, you guessed it, it's a LEFT hand thread,
which gets back to your original question:- why?

bleepe, I'm buggered if I know.
Now I wanna know why too!

Tuco replied:

Traditionally, and for safety reasons, flamable gases (LPG, acetylene, hydrogen, etc) use left hand threads while non flamable (oxygen, nitrogen, compressed air, etc) use right hand threads. Because of this safety factor it is impossible to install fittings to the wrong cylinder - eg in an oxy/acetylene set where there is one left hand and one right hand thread.

This has been the Australian standard for many years.

Then along comes "Primus" with their right hand threads on LPG cylinders - which goes against the safety standard! I have no idea how they were permitted to use the incorrect right hand threads on LPG equipment.

All other manufacturers, to the best of my knowledge, use the "Companion" left hand thread or the POL left hand thread for all LPG cylinder fittings.

Ray Bates replied:

Yes Primus do have to be different. I have noticed that some of their bottle are fitted with female threads. I believe they must be Americans

Stuart replied:

Just to make life a little easier, if you look at the fittings you will notice that nearly all left hand thread fittings have a notch around the circumference. It takes the guess work put of it.