The Stirling 8ft Single in 2½ inch Gauge
by Peter Gardner
Many of you will know of, and some seen, my Stirling Single running at
various tracks in the South: Leatherhead, North London, Staines, Reading, Sutton Coldfield,
New Romney and at John Llewellyn's track to my memory. I built the locomotive based on the
information contained within David and Charles' "The Stirling Singles" by K.H.Leech and
M.G.Boddy, which is long out of print but can still occasionally be found - at a price.
"Locomotives Worth Modelling" by F C Hambleton is also useful (also for NETTA).
The brief notes by LBSC are not much help as they virtually amount to a suggestion only
and for a very coarse approximation to the original. I had originally planned to build
one in 3½" gauge but the late Jack Davies persuaded me to try 2½"
As I had to produce my own castings, I though I might as well build two
Subsequently, requests from other Association members resulted in thirteen sets of castings being produced and disposed of.
However, to my knowledge, my first engine is the only one running and this is now 15 years old!!
My second locomotive has been at the nearly finished stage all this time - my wife regularly reminds me of the need to finish it.
I recently donated the patterns to the Association and the other day a
letter from Des Adley exposed the problem prospective builders have with the lack of constructional information.
I'm afraid that I rarely draw as a design exercise, only sketches and CAD if I want laser cut components, so I have no paper drawings done except for the main frame outline.
I sketch and measure and scale from illustrations then draw accurately straight on the metal.
However, as I have the nearly complete second loco to remind me of what's what, I have volunteered to compose a script, accompanied by sketches where needed, to help prospective builders.
Many 2½" gauge locomotives are quite heavily constructed and the technology is similar to that of a small 3½" gauge loco.
In contrast the Stirling, however, has much in common with a Gauge 1 loco, in fact conversion to gas firing has tempted me before now when I couldn't get a coal fire alight.
A few general notes about the model may be useful to start with so you know what you might be letting yourself in for.
Choice of Prototype
As the Eight Foot Singles were produced over a long period and many
were rebuilt several times there is a wide variation to the options available.
I chose to build No.1 as originally built because the trailing wheels are small and I could get away with one pattern for leading bogie, trailing wheels and tender.
The disadvantages are that this means a long run at the foundry (12 identical
wheels per locomotive) and, in addition, along with the small wheels there is a
short firebox that was later extended, together with the frames, only when the
larger trailing wheels were adopted.
If a later prototype or a re-build is modelled, this amounts to an additional 3/8" at the back of the frames and a longer firebox.
No.3 was proportioned similarly to No.1 but I believe that they were the only ones.
Any other differences become cosmetic in a 2½" gauge model, such as
splasher slots, safety valve cover shape etc..
You could opt for a late Ivatt re-build with solid splashers, squared off cab and domed boiler but for me the looks are totally spoiled.
Choice of Tender
There are two basic types of tender.
The early engines as originally built had Sturrock pattern with springs hidden between outside frames and later ones a more typical Stirling arrangement with outside horns and springs.
The Sturrock pattern is simpler to make, as it doesn't require scale horns or springs and applies to the early locomotives.
It seemed to me that the Stephenson's valve gear would be tricky in this gauge due to the limited clearance under the boiler bearing in mind that the over-scale bosses on rods can limit the valve travel unless the links are lengthened.
Instead, I opted for Allen straight link gear with box launch type links - a mistake.
I never managed to reduce die slip to an acceptable level and whilst it is satisfactory in full gear and one notch up, it is pretty poor
in mid gear.
This is in total contrast with my Netta, which will run in mid gear if not working too hard.
With the benefit of hindsight (experience) I would now fit slip eccentric with a dummy reverser to fool the rivet counters.
A surreptitious shuffle as you move the lever and you're in reverse.
The presents few problems apart from it's small diameter (2¼").
A couple of taps with a steel blank the same diameter as the drivers gives a little more clearance and the dents are hidden by the splashers.
I have managed without cladding and avoided paint discoloration by using good quality coach finish.
Whilst the grate may seem small, it is larger that that in my 3½"
After half a dozen failures to get started with my Invicta, I had to take lessons from the late George Williams before I could get a good fire
Too much paraffin on the charcoal and you end up with a full head of steam and yet no fire - an oil burner in fact. The secret is not to overload the
firebox and build the fire with dry charcoal then coal all in very small pieces a little at a time.
The tender hand pump on my loco feeds into the bottom of the axle
pump thus reducing the number of tender to loco connections and enabling the use of the hand pump to clear any stuck clacks.
LBSC usually specified an axel pump that was far too large for the locomotive and this needs to be avoided in such a tiny one as this or you kill the fire.
My rule of thumb, for all two-cylinder locomotives operating at 80 psi, is that the pump displacement is about 1/50th of a piston displacement (one stroke).
This is ¼ x ¼ inch for the Stirling.
I originally used a lazy cock rather than a bypass to save pipework and space. However, this can result in air being drawn in so that the pump then has to be primed (just as in full size) so I found space for a bypass but with difficulty.
This is where I had my real problems.
I originally planned to use a displacement lubricator at the front above the bogie.
I found, however, that the height restriction prevented good separation of oil and water and performance was erratic.
I tried various mechanical lubricator designs but still plagued by unreliability until I made a non-return valve using a tiny "O" ring from a discarded cigarette lighter which Ted Benn kindly let me have (his tip too).
The lubricator is of the fixed cylinder type but arranged horizontally.
The superheater is an arrangement I have used where space is limited.
It is a stainless tube with a splitter plate to divide the out and back paths.
More efficient than the concentric type, it is simple to make and is radiant but
with no need for brazed or welded joints at the hot end.
The regulator is push-pull and based on LBSC's needle in a hole with an "uncle Abraham spring" though with a 16 SWG operating rod, very little spring force is needed. You might expect that priming would be a problem with a small diameter domeless boiler but following the same approach as used in full size by Stirling, I have never had any priming at all.
The steam pick-up is by a large number of small holes running the length of the tube containing the regulator.
With that experience I always use "pepper pot" steam pick up in my locomotives - large water drops get trapped by the small holes but steam passes freely through the other dry holes.
This introduction was followed by 3 further articles, each supported by colour
photographs and CAD drawings. These, plus other useful supplementary data can
be supplied by the Association as a "Design Pack".
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