JasonB your photo tells it all. I can see now that there is a small pipe with a union nipple already attached – could have been turned from solid brass. That is secured to the pressure gauge by the union nut. It then becomes a ‘simple’ matter to extend that pipe probably by inserting it into a larger copper pipe.

This is possibly what Stuart Models did. The challenge is not to let solder get into the pipe bore and not melt the pressure gauge during soldering. Maybe I’ll make a CAD drawing of the arrangement.

Anyway, your photo and explanation has answered my basic question so thank you very much.

Thank you also to everyone who has responded to my request for help.

CAD Drawings for Small Pressure Gauge Syphon Connection

Thank you JasonB again for mentioning that pressure gauges commonly come with the long nipple. My pressure gauge – 3/4″ diameter – for my 3/4″ scale Bassett Lowke Burrell model (1950 design!) has just arrived and it has a long union nipple of 3/32″ diameter, as shown in JasonB’s photo above.

I have now made two CAD drawings for the small pressure gauge to syphon connections one using standard dimension pipe fittings and one with the long 3/32″ pipe union nipple. As a thank you to all who have helped me in the post and also to assist others who might have similar requirements I am making these freely available for download.

These are small sized images of the full sized CAD drawings:-

The full sized CAD drawings can be downloaded by clicking here.

Once again many thanks to everyone who has helped me with this query.

PS CAD Drawings for the Banjo Connection

Since I’m going to make the syphon myself including a 1/4″ threaded union to screw into the boiler bush I’ll be making CAD drawings of these components too and will made those freely available for download too.