Thread Naming / Designation

[Brian Reay]

Not really model engineering related but I suspect someone here may have
the expertise or have come across this:


In modern motorhomes/caravans, the LPG regulators are mounted on the
vehicle, rather than the gas cylinder. A pig tail (rubber or stainless
steel) is then used to connect to the LPG cylinder (or cylinders,
possibly by some change over arrangement). Most people use Propane and
the cylinder end tends to be a 'POL' connector but there are others,
including a 21.8 LH thread generally known as the Butane connector,
although it is used for Propane as well !

The other end, ie the regulator (end, we can ignore any change over
arrangement), is most commonly known as a W20 although sometimes an M20.

I'm trying to find out want kind of thread the W20 is.

Looking on the 'net, it seems the W20 and M20 aren't the same but will
'mate' well enough, even for gas- the thread isn't part of the seal,
that is via a washer (rubber I assume).

The M in M20 suggests a metric thread and while the W could, perhaps,
suggest Whitworth, I thought the normal shorthand was BSW, plus I've
never heard of a metric Whitworth thread.

The 'nut' part of the female fittings all, at least all that I have
seen, have small notches in the edges.

Others, like myself, who have investigated this from the
motorhome/caravan world seem to have lacked the expertise in threads etc
to nail this mystery. While we can, and do, tend to simply buy the
correct fittings etc, I'd like to get to the bottom of it.

Can anyone help, please?

(Things get even more complicated if you want to connect to an European
gas cylinder as some have different connectors on the top but that is
another can of worms. You can't get UK gas cylinders in Europe.)

[Gareth's was W7 now W10 Downstairs Computer]

Post by Brian Reay
Not really model engineering related but I suspect someone here may have

As always, you haven't got a clue.

Leave well alone, gas installations can kill, and not just you.

[Chris]

Post by Brian Reay
Not really model engineering related but I suspect someone here may have
In modern motorhomes/caravans, the LPG regulators are mounted on the
vehicle, rather than the gas cylinder. A pig tail (rubber or stainless
steel) is then used to connect to the LPG cylinder (or cylinders,
possibly by some change over arrangement). Most people use Propane and
the cylinder end tends to be a 'POL' connector but there are others,
including a 21.8 LH thread generally known as the Butane connector,
although it is used for Propane as well !
The other end, ie the regulator (end, we can ignore any change over
arrangement), is most commonly known as a W20 although sometimes an M20.
I'm trying to find out want kind of thread the W20 is.
Looking on the 'net, it seems the W20 and M20 aren't the same but will
'mate' well enough, even for gas- the thread isn't part of the seal,
that is via a washer (rubber I assume).
The M in M20 suggests a metric thread and while the W could, perhaps,
suggest Whitworth, I thought the normal shorthand was BSW, plus I've
never heard of a metric Whitworth thread.
The 'nut' part of the female fittings all, at least all that I have
seen, have small notches in the edges.
Others, like myself, who have investigated this from the
motorhome/caravan world seem to have lacked the expertise in threads etc
to nail this mystery. While we can, and do, tend to simply buy the
correct fittings etc, I'd like to get to the bottom of it.
Can anyone help, please?
(Things get even more complicated if you want to connect to an European
gas cylinder as some have different connectors on the top but that is
another can of worms. You can't get UK gas cylinders in Europe.)

You could use a thread gauge to measure the pitch, then look it up
in thread tables. Not much experience of the type, but probably some
variant of bsp, gas thread or similar...

Chris

[Brian Reay]

Post by Chris

Post by Brian Reay
Not really model engineering related but I suspect someone here may have
In modern motorhomes/caravans, the LPG regulators are mounted on the
vehicle, rather than the gas cylinder. A pig tail (rubber or stainless
steel) is then used to connect to the LPG cylinder (or cylinders,
possibly by some change over arrangement). Most people use Propane and
the cylinder end tends to be a 'POL' connector but there are others,
including a 21.8 LH thread generally known as the Butane connector,
although it is used for Propane as well !
The other end, ie the regulator (end, we can ignore any change over
arrangement), is most commonly known as a W20 although sometimes an M20.
I'm trying to find out want kind of thread the W20 is.
Looking on the 'net, it seems the W20 and M20 aren't the same but will
'mate' well enough, even for gas- the thread isn't part of the seal,
that is via a washer (rubber I assume).
The M in M20 suggests a metric thread and while the W could, perhaps,
suggest Whitworth, I thought the normal shorthand was BSW, plus I've
never heard of a metric Whitworth thread.
The 'nut' part of the female fittings all, at least all that I have
seen, have small notches in the edges.
Others, like myself, who have investigated this from the
motorhome/caravan world seem to have lacked the expertise in threads etc
to nail this mystery. While we can, and do, tend to simply buy the
correct fittings etc, I'd like to get to the bottom of it.
Can anyone help, please?
(Things get even more complicated if you want to connect to an European
gas cylinder as some have different connectors on the top but that is
another can of worms. You can't get UK gas cylinders in Europe.)

You could use a thread gauge to measure the pitch, then look it up
in thread tables. Not much experience of the type, but probably some
variant of bsp, gas thread or similar...

Thank you for the suggestion. However, based on what I've gleaned from
various sources so far, the differences between M20 and W20 are more
subtle than pitch. The basic dimensions, if I can use that a crude
collective term, for the key features of the thread- inc pitch and
diameters seem to be very close. (The two 'mate' quite happily,
certainly well enough so the part of the connector which seals is gas
tight.) While a measurement with a vernier or micrometer would probably
show any difference in the overall (major?) diameter, would a visual
difference in the pitch be apparent? Others who have also researched
this have mentioned angles, 5 and 7 degrees are mentioned, but without a
context, the 2 degree difference could mean anything.

While, in practical terms, I (and others) can simply buy the appropriate
fittings and they will work safely, it is just one of those things I'd
like to get to the bottom of. It isn't something you need to work on
often, more something that crops up when you need to replace a hose etc
at the end of its life (typically 10 years for a rubber one). My
interest was stimulated as our new motorhome has a clever system which
allows you to run the gas heating while driving. While I don't use it, I
read up on the details etc and it includes some special, extra, fittings
which cut off the gas if the hose rupture in an accident, plus there is
a 'crash sensor' which cuts of the supply even if there is no rupture.
(Normally, you simply turn off the gas at the cylinder(s) before
driving, which I still do, not least as most of our trips are via the
Channel Tunnel and they demand it.)

[pyotr filipivich]

Post by Brian Reay

Post by Chris

Post by Brian Reay
(Things get even more complicated if you want to connect to an European
gas cylinder as some have different connectors on the top but that is
another can of worms. You can't get UK gas cylinders in Europe.)

You could use a thread gauge to measure the pitch, then look it up
in thread tables. Not much experience of the type, but probably some
variant of bsp, gas thread or similar...

Thank you for the suggestion. However, based on what I've gleaned from
various sources so far, the differences between M20 and W20 are more
subtle than pitch. The basic dimensions, if I can use that a crude
collective term, for the key features of the thread- inc pitch and
diameters seem to be very close. (The two 'mate' quite happily,
certainly well enough so the part of the connector which seals is gas
tight.) While a measurement with a vernier or micrometer would probably
show any difference in the overall (major?) diameter, would a visual
difference in the pitch be apparent?

Depends on how clean things are, how steady you can hold things,
and how good your eyes are.

So, "yes" for some values of "yes, you can."