The Heart Of The Matter
by Dick Moger

Richard Finlayson has asked me to provide a few more details of the novel arrangement of the cylinder block used in the Dee design as published by The Gauge One Model Railway Association (G1MRA). The book went on sale in February 2001 and has already stunned its authors by selling over two thirds of the print run in the first few months.

 

While it is true that there is a pent up demand for such comprehensive home build manuals, the interest in this case no doubt stems from the curiosity arising from this unusual approach to cylinder design. The genesis of the idea comes from the "Twin Insides" G1MRA technical Forum of 1993. At this Forum, John Van Reimsdijk outlined his approach to the subject and it is typical of his thinking. JVR is the Matisse of the steam world… why bother with complexity, when you can perform the same function with minimal resource? We have taken this idea and modified it to suit the requirements of a classic Edwardian inside cylinder 4-4-0, but clearly the idea applies equally to outside cylinders.

But why would you want to do it when so many cylinder designs abound and all seem to function? Not all the advantages were clear when we started with this project, (Small p!), although it was clear that less work was involved, simply by reducing the number of components and machining operations. The ports are simple drilled holes, 1/16" for the steam ports and 1/8" for the exhaust. The valves are no longer square buckles with a recess, but simple round brass, shaved and notched. A steam chest is no longer needed, thus saving the additional length normally required for the bolting surface and gaskets. In fact, Dee has a cylinder block of only 1" in length, which does wonders for the free movement of the bogie as well as the appearance.

Strangely, what was originally considered a downside can now be viewed as a positive asset. This concerns the valve timing, and it is quite apparent that this could potentially be difficult to perform given the limited view of the ports themselves. I was unhappy with an original proposal to mill out the unused top half of the port drillings to improve visibility and then machine a guide slot to stop the valve rotating.

This seemed completely at odds with the minimalist approach of the original idea, so I pressed on until I reached that stage and found the problem resolved itself in the most satisfying manner. If the top portion of the port drillings are left intact, a stud can be inserted into the centre drilling and backed off to allow the valve to lift, but also prevent rotation.

To set up the timing, you remove the stud, flip the valves over on their backs and use the TOP portion of the drillings to set the timing. Not only is this easy to see, but you have the luxury of using the edges of the exhaust cavity in the valve to set up accurate valve events which is something that you cant do with a flat slide valve.

Another unexpected effect of this design is that the actual opening of the valve is marginally less critical than it is with square cut ports, as the opening is now gradual.

You get slightly reduced 'smokestack talk' as a result, but since this is a proper Stephensons locomotive with a working front end, it will still out-chuff any of the usual commercial closed flue/open smoke-box gas fired designs.

 

 

Yet one more benefit is that the feathered entry of steam due to the round ports seems to make the driving wheels less sensitive to slip when heavily loaded.

The only real disadvantage with this approach is that more care is required in lapping in the round valves. The slightest remnant of a burr caused by machining in the exhaust cavity will prevent the valve from sealing. We recommend a simple method of doing this in the book and have not come across any difficulties to date. You will note that the valve itself has the top and sides flattened to allow the valve to lift in the presence of trapped water. A 1/16" diameter valve rod passes through the 3/32" diameter hole drilled up the centre of the valve to facilitate this lifting, the valve being held in position by nuts, but free to move vertically.

JVR's approach differs in that a single valve bore is used between the cylinder bores, with the depth of the valves themselves very much reduced. A device fitted to the end covers keeps the valves upright and stable. To date, we have had no indications of wear problems despite the very considerable mileage undertaken by the models concerned, and the compression caused when reversing the slip eccentric is excellent.

To summarize the method then: It has the advantage of simplicity and ease of construction combined with a reduced overall size that allows its inclusion into the designs of the smaller, earlier locomotives without compromising either appearance or performance. It is equally adaptable to outside cylinders and combines the look of a piston valve with the more easily achieved long life of a slide valve. It is noticeable also that the reduced surface area of the valve (Half that of a conventional slide valve) results in a very free running engine.

To find out more about the design, consult the G1MRA website at http://www.gaugeone.org