January 2019: the Wrightscale 16mm Wren

We wish all friends and fellow 16mm enthusiasts a happy and prosperous new year! May our wishes come true and may our worst fears not be realised. It is also healthy to recall that sometimes it’s better to face the fear in order to realise our dreams.

16mm Wrightscale Wrens

We are pleased to say that the latest batch of Wrens is making progress. They are beginning to look like locomotives. Better still they have reached the steam test stage, that is, they have nearly become proper locomotives.

A number of stages mark this journey.

The first one of course is thinking. Well was it said ‘measure a thousand times, cut once!’ NOTED The Wren was built to Imperial standards so metric equivalents are only approximate.

Kerr Stuart Wren nameplate Photo copyrioght MD Wright

The prototype Kerr Stuart Wren - with a wheelbase of a mere 3’ (90cm) designed to run on 2’(60cm) - was very small. This was intentional. Competitors abroad, especially Decauville, Jung and Henschel, were world leaders in compact locomotives and the British couldn’t afford to miss out. In 1905, Kerr, Stuart started to adapt an existing design, the 0-4-0 Buya Class to produce a locomotive to run on 2’ gauge. The early Wrens were built with low-slung boilers and cylinders. They had Stephenson’s reversing links. The wheels were one-piece with a group of four holes. This was a problem. Because of its dainty weight – 3 ton 7 cwt (just above 3 metric tons) - the locomotive tended to ‘sit’ on its back wheels when starting off with a heavy train. This led to uneven wear. Soon, the wheels had steel tyres fitted, making them slightly easier to re-machine.

This 16mm Wrightscale Wren is a model based on the new type built after 1915

In 1915, the ‘new-type’ Wren was introduced. It benefited, dare we say, from improvements inspired by Continental practice. The boiler barrel was lifted 7 ½ “ (19cm). This made it a little less vulnerable to debris on the track, but also made for a more robust frame. The cylinders were inclined, again, allowing greater clearance between them and the track. Proper cylinder drain-cocks were possible. Previously they drained through the front covers, an arrangement which encouraged a cloud of steam around the front and corrosion.

In all, 58 of the old-type were built and 104 of the new-type before Kerr, Stuart closed. Prototype size Wrens are still being produced thanks to the engineers at Statfold Barn – but that is a story in itself! In spite of improvements, the old-type continued to be requested. Perhaps some proprietors had no truck with nasty foreign innovations.

We chose to model the new-type for various reasons. One is the light and airy look. Another is that there are more surviving new-types, therefore they are better known. They would therefore be more popular with you, our market, and there is more information around. The last is personal. We fell in love with ‘Pixie’ at Leighton Buzzard on a family expedition back in 1980. The Wren’s history and our choices inform everything about the model.

Part-built 16mm Wren locomotives in the workshop, January 2019

The frames for each locomotive are the first part. This includes fitting the axles and the wheels without which there will be no movement. As becomes a new-type model, the frames are relatively tall. They also have a prototypical (sort of!) strengthening web. We know that the completed model will be treated with respect, but accidents do happen, on 16mm lines as well as 2’ gauge!

Here has to follow a slight disclaimer. The frames and strengthening web vary slightly from the prototype – not being completely faithful 1:19 scale models. In scaling down, some liberties have to be taken. The overall dimensions, if scaled up, would be too robust. Look at it this way. A healthy, functioning baby is not a scaled-down version of a healthy, functioning adult. In the same way, producing a live-steam 16mm scale locomotive requires some trade-offs.   

Next come the cylinders, a surprisingly ticklish job as this stage includes fitting the connecting rods. Next, the valve chests and pistons are fitted. This is science and art combined. Each piston has to ‘feel’ loose in the bore but, of course, not too loose. As mentioned above, for the best reasons, the prototype cylinders are inclined, which adds to the joy of building!

Next is making up the valve-chests and steam Tee then the fitting of each connecting rod. It is vital to get it right. The valve gear can now go together. At this point, the Hackworth link must be fitted. As we saw before, this small alteration made a big difference to the Wren. The reversing shaft was moved to a place above the driving wheels, a reason why the locomotive has its airy appearance.

Batch of 16mm Wrightscale Wrens in the workshop January 2019

Fitting the link requires work with a file. ‘Square up and bevel gently’ is the advice – the artist’s hand is needed here. The valve-gear assembly must move freely but with very little shake. Only then can the valve-rod be fitted. A little art is needed here to avoid the opposing evils of sloppiness and binding.

Now the valve events can be set. Oddly enough, there is something of the skill of riding bike here. While one side rests, the other comes into play. There should be two points in the complete cycle where the ports on both side are open equally. Here, the analogy with a bike breaks down. A locomotive can operate in reverse; a bike doesn’t. The valve ports must have their equal points both when going forward and in reverse. Actually, as with the prototype, you do want the best running forwards. You may achieve this by beginner’s luck, or by careful adjustment.

Wrightscale16mm  Wren in green livery

So far, the locomotive has been put in reverse by holding it up-side-down and reversing the motion. This is not, to put it mildly, prototypical. Now the reversing lever can be fitted, then the saddle of the boiler. Once the boiler is fitted, the locomotive begins to look recognisable. Better still, the locomotive is on the road towards its first steam test. Stand by for further announcements!