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Gas Threads Explained In Plain English.

Article by Peter Smith - Caravans Plus

How to measure the gas/water fittings you need

Gas fittings and hoses used in Australian Caravans and Motorhomes use a number of different threads. They may seem hard at first, but a short read will give you enough information to purchase the correct part.

The following information is educational and general in nature only. Licensed Gas Fitters are required to install gas fittings into RVs.

All gas components on this website that have threads also have a list of other components that will connect to them. They are listed below each product description. This feature allows you to ensure all components from gas cylinder to gas appliance are compatible.

1) BSP (British Standard Pipe)

BSP threads are the most common and can be used for gas and water. The sizes are imperial.

To get a 100% seal with gas you use a yellow thread or Liquid thread seal. You can use white or yellow for water.

When used for water the most common one you will know around the home is on your garden tap. Most garden hose fittings come with a 3/4" BSP to 1" BSP adapter.

Gas Threads Explained

The size quoted for threads is indicative of the internal bore size, not the diameter of the thread.

Gas Threads Explained

Click here to print a Thread Sight Guide to lay your thread on.

Click to display an actual size thread template

Above: If you have an existing thread that looks like those above, you need to measure the outside of the male thread as shown. The male thread is generally tapered so that it gets tighter when screwed in.

9.5mm measured OD is Male BSP 1/8"
13mm measured OD is Male BSP 1/4"
16mm measured OD is Male BSP 3/8"
21mm measured OD is Male BSP 1/2"
26mm measured OD is Male BSP 3/4"
37mm measured OD is Male BSP 1"

Above: If it is a female fitting, measure the inside to determine the actual name. Measured sizes are shown as approx. mm

8mm measured ID is Female BSP 1/8"
11mm measured ID is Female BSP 1/4"
14mm measured ID is Female BSP 3/8"
17mm measured ID is Female BSP 1/2"
21mm measured ID is Female BSP 3/4"
31mm measured ID is Female BSP 1"

2) SAE 45 degree flare

The male fittings are easily identified by the 45 degree flare at the end. These screw into either a SAE nut on the end of a copper pipe or a gas hose.

When used with copper pipe, the pipe is flared to 45 degree. The nut will not fall off and the copper section creates the seal between the nut and SAE 45 flare brass fitting. No thread tape is required with these fittings.

Gas Threads Explained

The Outside Diameter of the thread can be measured to give you the correct name of the fitting.

11mm measured OD is Male SAE 1/4"
13mm measured OD is Male SAE 5/16"
16mm measured OD is Male SAE 3/8"

Measure the Inside Diameter to get the correct name of the nut. The nuts always swivel. They have an inverted flare inside.

10mm measured ID is Female SAE 1/4"
11mm measured ID is Female SAE 5/16"
14mm measured ID is Female SAE 3/8"

The nut shown is a 5/16" nut because it fits on a 5/16" pipe.

The recommended pipe dia. for a typical gas installation within a Caravan or Motorhome is 5/16" copper pipe.

If you have a gas BBQ, Gas stove, Gas HWS, Gas heater and large Gas fridge - you may need 3/8" pipe, but your gas fitter can determine this.

3) POL fittings

Used on larger gas cylinders these have a Left Hand thread. Many male fittings have a rubber o-ring and must not be over tightened.

Gas Threads Explained

4) Primus gas cylinder threads

One notable difference with this thread is that gas will only flow after the fitting is fully inserted into the cylinder.

Gas Threads Explained

5) 3/8 inch Left Hand gas cylinder threads

Often called Companion but used by other brands as well.

Gas Threads Explained

6) Bayonet fittings

Used for removable gas appliances. The female fitting is used to supply the gas and comes in one size only. A rubber cap should be placed on these if subject to dust or dirt.

caravan bayonet fittings

7) Inverted Flare 1/4

This is used at the end of gas pigtails that connect to a change-over value. The seal does NOT require thread tape.

Gas Threads Explained

Was this helpful?

(54 votes)


  • comment-avatar


    Useful information but would be even more so if the thread pitches were also listed.

  • comment-avatar


    Very useful, always been confusing working out difference between gas,air and many other threads

  • comment-avatar

    Colombia vm tröja

    Maintain the exceptional work !! Lovin' it!

  • comment-avatar


    Hi I own a Weber Baby Q and I have a camper trailer that has a female bayonet fitting. I would like to connect the Baby Q to that fitting so that I can use the on board gas facilities rather than carrying a separate gas bottle. Is there an adapter that has a female POL fitting on one end and a male bayonet fitting on the other? Thanks Nelson

  • comment-avatar


    The Weber range use a male 3/8 flare connection, not a POL. The POL is the fitting used on a gas bottle. What you need is a male bayonet to female 3/8. Search on the web there are plenty available.

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