Banjo Bolt and Pressure Gauge

A banjo bolt and pressure gauge for the small vertical boiler. Also, I made a siphon tube, this protects the gauge from the very hot temperature water and steam.

Banjo Bolt

banjo bolt machining

The banjo bolt was machined from 1/2 inch brass round stock. The threaded section is 1/4 inch x 40tpi and this bolts into the side of the boiler. The bolt has a 2mm diameter hole in from the threaded end, this extends up to the necked section of the bolt between the thread and the head. This necked section then has a hole drilled in at 90° to the axis of the bolt.

The banjo bolt was then fixed into a hexagonal ER25 collet block. This allowed me to simply machine the bolt head without the need for a dividing head.

milling flats on the banjo bolt head

I set the milling bit to machine 1.55mm depth, this then would cut a 3/8 inch hex head from the 1/2 inch round bar.

I then machined the flat, unclamped the hex ER block in the vice and rotated it one division on the hex section. The key to getting accuracy is to ensure the edge of the ER collet hex block bottoms out on the flat of the vice each time.

hex headed banjo bolt

After 6 machining operations I had a perfect hexagonal head on the banjo bolt.

In this image you can also clearly see the 2mm diameter hole drilled in from the end.

Below you can see the finished banjo bolt sat on hexagonal ER collet block.

banjo bolt sat on hex ER collet block

Siphon Tube

annealing copper tube

The pressure gauge on this boiler is tiny and the olive fitting is designed to take 1/16 inch OD copper tube.

The first thing for the copper tube is to form the U-bend. Hence turning it into a siphon tube. This means steam will condense in the U-bend and thus protect the gauge from the very high temperature steam and water.

In order to bend the tube one of the first jobs was to anneal the copper.

To anneal copper you just need to heat it up and then let it cool. To heat it up I used a methylated spirits burner. This will heat the copper up quite rapidly to a dull cherry red.

Bending the siphon tube

To ensure the copper tube doesn’t collapse whilst I’m bending it I machined a simple jig.

This jig is a piece of brass bar with a semi-circular 1/16 inch diameter groove. As pointed out by a friend it is better to machine the groove to a depth that matched the diameter of the tube. This then allows you to run a flat plate around the pipe and jig and ensures the pipe cannot expand sideways.

If it cannot expand sideways then the pipe will not go oval/flat whilst being bent.

siphon tube

The siphon tube post bending it to a 9mm radius.

The siphon tube was then cut to length with a saw and both ends filed and deburred. I use a small drill to deburr the centre hole at each end.

Note: take care and blow the pipe through before fitting.

I silver-soldered the fittings to each end of the pipe. I used silver solder paste and a blow torch to get a very quick and neat finish. The size of these parts is more like jewellery and so silver solder paste is appropriate.

The banjo bolt and pressure gauge all in place on the boiler.

pressure gauge, siphon tube and banjo bolt all fitted

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