16mm Wrightscale Wren

The Wren and calm

Running your 16mm locomotive is fun and the opportunity to socialise is good too. Above and beyond these benefits are others. Some are quite unexpected. A live steam locomotive may not work or may be capricious. Far from causing anger and frustration, this challenge can bring unexpected benefits.

16mm Wren locomotive built by MD Wright

A customer has just approached us with a problem. He won’t mind us quoting him. ‘It has had numerous runs, some more successful than others but an absolute joy when it is doing well. The motion is silky smooth with a healthy beat and slow running achieved’

‘But’ and this was a big ‘but, the flame popped in and out of the boiler like a ticking clock’ This exposed the buffer beam to the flame. The solder melted and it fell off. The paintwork was not improved as it too softened when heated.

Oddly enough, there is good news in this sad story. What is needed is the limited and realistic exertion of control. As the professionals say ‘Figure out what constructive actions can be taken and then take them.’

Oddly enough, whether you achieve your required goal or not, your anxiety levels will fall!

Malcolm will go to his shed (the nice warm new one). Hours just melt away as he figures out the problem. When he finally emerges, he is calmer than before. He has of course not been able to work on other projects, so there is a cost, ‘trouble-shooting’ is in itself a therapy.

Front of a Wrightscale 16mm Wren. Flames popping in and out are not a good look Photo Malcolm Wright

We come back to the popping flames. They may seem like a small issue but the results – flames shooting out of the smokebox, melting solder etc, can be all too apparent. First of all, he had to look for a cause.

The performance of the gas jet depends on the very small jet hole being clear; the iron laws of physics demand that the aperture is small so that a jet effect is enjoyed. Any material that lodges in the jet hole will alter the path of the gas into the burner. If the jet fails to be co-axial, entrainment of air will be very hit-or-miss making the burner often impossible to light, slowing down the speed of the gas jet and therefore rendering the gas-air mixture suboptimal. This leads to it being difficult to light, slow to raise steam, burning occurs in the smokebox, etc.

The most common reason for these problems is the presence of minute particles of copper oxide. This is because the gas system is fully silver soldered and has to be pickled on completion. The sulphuric acid converts most of the copper oxide formed into copper sulphate and therefore a small amount of detachable residue can be left inside the tank or pipe to the jet.

There is good news. This is a problem that will disappear but might take quite a time to do so.

To solve the problem, the main tool to use is the jet pricker wire. Use a watch-maker's screwdriver of the correct size to undo and tighten the gas jet screw. The screw does not need to be very tight though use the check described below.  

Wrightscale 16mm Wren showing the jet cleaning screw, Photo MD Wright

Thus, if the burner becomes difficult to light, clear the jet using the pricker wire. Do nothing else apart from checking that the screw has been tightened just sufficiently.

When the engine is on the track, raising steam, just waft a flame near the back of the burner to check that there is no gas leaking from the screw. If it is, blow it out, tighten the screw a tiny bit, retest. If it is still leaking, replace the o ring.

Excess gas coming through the jet may cause ‘popping’ If there are no leaks round the jet and its holder and the problem is minimal, it should disappear when the gas has settled down and is burning properly.

  In this particular case, the problem was solved. The Wren was soon running like the proverbial Swiss watch.

Enthusiastic Wren-builders may wish to solve another problem.  The gas control valve may leak. In this case, it should be repacked with PTFE tape – quite generously. Then the gas flow is ‘off’ with one turn; the valve is consequently tighter to operate. For hose who have bought a ready-to-run Wrightscale Wren, consult Malcolm before you think of doing this operation. He is happy to do this and all you have to lose is your postage (and the cost of the materials you would have had to buy anyway). 

Another view of teh 16mm Wrightscale Wren Photo MD Wright

If a paint-job is needed, for the tank and buffer beam, start by stripping them back and before applying a recommended etch primer. Again, check with Malcolm before doing this job. He may well offer to do this for the cost of materials and postage.

If a Wren just needs retouching, get a small spray can of the correct colour of Halfords Car Paint. Clean off the area with Ronsons Lighter Fluid before starting, spray some paint into the lid and use a small paintbrush to retouch.

We hope that you are now breathing more easily. Anxiety isn’t a cruel psychological anomaly but a good motivator. You have faced the problem, analysed it, done what you can and recognised what you can’t. This is called living!