Building the Allchin traction engine

Last updated 14th March 2011

Starting the ' Journey ' ...........some background information

(Further down the page are more detailed descriptions of the component parts)

With two locomotives and a number of stationary engines under the belt we looked around for a more challenging, long term project. For years we had enjoyed the sight, smells and sounds of traction engines, redolent of our childhood when we watched what seemed like ' 'Monsters ' towing trains of trailers. The drivers were always friendly when we chatted with them whilst taking on water and on occasions allowed us up onto the footplate or manstand. The interest and memories persist.

Prototype The Alchin build to 1 1/2" scale was serialised in ' Model Engineer ' magazine, the author, an accomplished engineer WJ.Hughes' descriptions of the production techniques made the proposition appear to be within our capabilities, and so the die was cast.
The book on building the Allchin, Royal Chester was purchased at the Model Engineer exhibition and the first few castings bought from Messrs Reeves. So started what became a long and sometimes arduous build, particularly for a concrete engineer with self-taught metalworking skills !

On the shelf, part built, for sale.

It doesn't surprise us that one frequently sees part-built Allchins and sets of part-machined castings for sale at exhibitions, or advertised in the model press, unfortunately adverts in the latter case frequently mention failing health as a reason for the sale.

At a recent exhibition I watched a fellow visitor purchase the book and took the opportunity to mention what a task he was letting himself in for......I am sure that he didn't believe me when I said the I had spent 6 years on my model...he might not take that long, but it may well take him two or three years, depending on his other commitments !

In retrospect

Looking back over my activities, I learned a great deal, honed my skills and have now a model of which I can be reasonably proud. I work slowly and often remake items that do not meet my own set standards, Not the least of the challenges was to understand how the engine worked and in the back of the mind was always the question....' when finished, however long it takes, will it work ? ' Ready to go.

The finished model measurements are as follows, Length 24" (81 cm ) Width across wheels 12" (31 cm ) Height 18" (48 cm )

I keep a log of my modelling activities and see that I made a start on the chimney, fashioned from a piece of reinforcing bar for concrete on the 1st of February 1996. The model was completed with the addition of the second-shaft cover on the 17th April 2004 after a total of 1358 hours total worked.

I took a one year ' Sabbatical ', for other modelling activities, ( Building a radio controlled boat ) between 1998 and 1999 making the build a 7 year effort overall. Of course the overall period included making the various jigs and fittings to construct parts such as the wheels, forming the tender plates, drilling for the pinions in the compensating ring ( Differential ) etc.

Gleaming new boiler I have to add that although I had built the boiler for ' Tich ' it was apparent that as was the case in building the 2 1/2" scale model locomotive ' Pixie ',the mass of copper in the Allchin boiler would demand greater heat input than I could achieve with my equipment. The ' Allchin ' boiler was thus sublet, more detail later....

Part by part.........................

I started with the smokebox end, and the chimney in particular, as a discrete part of the model which would establish whether my skills would adequate for the task. For some long time, just the front end mounted on a stand provided the motivation to carry on and complete the work !

The chimney was a problem for me, mainly due to my choice of material ( High tensile reinforcing steel ) Once the appropriate lathe speed was established the turning and boring was complete using Tungsten tippped tools. The seam was milled and dummy rivets inserted to simulate the rolled steel of the prototype. The chimney cap casting from a proprietary supplier failed to meet the sculptor Henry Moore's saying that ' The required form is in there, all the sculptor has to do is reveal it '

It was remarkable thet the supplier having produced capping castings for many years had not been made aware of the failings of this product. Their new pattern avoids the problem for future builders.



Here the spuds (wheel attachments! ) were kept. The spud pan and front-axle assembly present little difficulty and with completion of the perch bracket it really felt as if progress was being made.


The smokebox and smokebox door next on the list. The weld in the rolled tube for the smokebox called for a tungsten tipped tool to be employed in turning the outside diameter, otherwise no problems. The smokebox angle demands careful turning for fit and the rivetting is awkward, calling for an angled snap. The door casting needed only light turning to preserve the cast-in makers name. The hinge plate called for care in turning and fitting to the door section. Hinging and pinning is tricky for a novice although a final pass using a reamer with the door clamped into place overcomes any initial small innaccuracy .
Being fairly pleased with progress and product at this stage I mounted it on a order to provide motivation !


Hind wheel hub

The wheels provided few problems although requiring a substantial amount of work in fabricating the hubs Particular care is required in machining the seatings for the spokes in the hind hubs, due to their oval shape. Rather than sweating, Araldite was employed to secure all the hub elements. We are quite prepared to take advantage of modern technques even whilst building models of early in 20th Century prototypes.

Turning the rim 'tee' sections called for care to ensure matching pairs for later combination by the application of the strakes.

Completed hind wheel A great many rivets are required and initially iron rivets made for hard going. Copper or brass would have been easier to form. The purchase of pressed spokes is recommended to avoid otherwise tedious, time consuming milling operations.


The spokes are individually installed with the rims positioned on turned wooden jigs, In the case of the hind wheels due the oval profile of the hubs this is essential in bending the spokes to achieve a good fit at hub and rim.


The hornplates were set out and the many holes drilled in accordance with the, now,much thumbed drawing. At this stage the relative positions of the holes for bearings to the three shafts and the hind axle are critical and need checking and rechecking prior to setting-up in the lathe for boring. Many of the rivets are dummy rivets, and the legend at the bottom of the detail identifies various hole sizes which serve different purposes in final assembly. Shot of drawing courtesy of Nexus Publications


The bearings in general are rather complicated in form and again the drawings need careful interpretation when setting out, The location of the various bearings on the hornplates being governed by the diameter of the gear wheels and critical to final engagement ( Mesh ) of the gears, no room for error here!

The hind axle provides interesting turning work together with milling of keyways, whilst not truly to scale we followed our practice of using the nearest stock- sized material, in this case silver steel.


The tender body and hornplates can be united at this stage, with the hind axle inserted to ensure all being true. The moment of truth as regards shaft bearing location is yet to come


The crankshaft provides considerable excitement. Setting out for, and maintaining accuracy when using two centres to achieve the throw is, for the novice, a demanding process, only matched by work on the ........


The second shaft with intensive work in machining the splines to allow the gears driving the third shaft for gear changes to be shifted. We have to admit to hand-finishing the splines on the milled shaft to match the keyways in the change speed spurwheel centre.


The third shaft is the tranfer shaft, carrying the drive to the compensating centre. Mounted at one end is the band brake drum. Braking is by winding-on a steel belt with wooden brake shoes. The brake shoes are turned on a faceplate and subsequently cut into individual pads.


The compensating centre and spur ring assemblies are for the amateur, probably the most demanding of all the parts to construct. They provide the basis for the differential action and for the winch winding drum. The pinions have to be absolutely correctly positioned and the drillings for the driving pins and differential locking pins must provide for alternate insertion according to the driving conditions. Accurate jigs are essential to success here


The cylinder block is a major item of construction, the drawing needs careful attention and it is worthwhile spending time on preparing a method statement. This will ensure that datum chucking and jigging points are maintained, The cylinder and liner turning and boring then insertion of the liner are probably the most demanding and critical items in the whole build!


The Cylinder Liner requires to be set into the cylinder block to form an annulus round which steam from the boiler passes to enter the regulater. Freezing permits its introduction into a warmed block. Return to normal temperature ensures a seal. Here the ready bored liner is held on a mandrel for the external diameter to be sized.


Trunk guide construction provides interesting turning practice! It's function is, , to translate, via the crosshed, the linear action of the piston into radial action at the crankshaft. Here a check is made that the valve rod will line correctly with entry to the valve chest which has yet to be finally seated on cylinder block.
Cylinder fixing hole angles are difficult to establish. A wooden model of the boiler, (then yet to be received from the maker) was made. The cylinder block set on this in the drilling machine was rotated and the previously marked holes were thus drilled axially to the boiler. The dummy boiler was useful in other operations and in an early mock-up with tender and smokebox
The cylinder block and trunk guide are fixed to the boiler by 5BA stainless steel bolts into tapped holes in the boiler. Tapping copper can be tricky and one bolt sheared! Fotunately the stub could be drilled and driven through into the boiler, then recovered via a bush...phew. Drilling and tapping a 600 boiler can be traumatic


Eccentrics and valve gear are critical elements in the operation of the engine ( The ' engine ' in this case is the cylinder block etc. which sits on top of the boiler ) the shells are bored and set on a mandrel offset in the chuck for finishing the periphery. The outer member is split into two halves and soldered back as one piece.After drilling for connecting bolts it is bored to match the eccentric then unsoldered.
The governer with it's 26 individual pieces approached watchmaking as far as this ham-fisted concrete engineer is concerned ! Fortunately a friendly musical instrument maker kindly made a jig for sizing and drilling the spring steel, a task which had seemed formidable thus simplified!
Water pump components require machining to very close tolerances. Construction is almost a seperate project, The casting has to be fettled, the various waterways bored and valve seatings machined. The cylinder and piston produce with a good fit and the eccentric providing drive from the crankshaft prepared. Just as in modern automotive practice, The traction engine manufacturer employed the crankshaft to provide drives for pump and valving


The manstand is quite a congested spot


The toolbox and running board provide a welcome opportunity for some woodworking practice. The local saw mill ripped me out a piece of prime beech from a huge plank ( For just One Pound ! ) People have suggested that my effort here, particularly the brass fittings. may qualify me as a coffin maker !


Steering gear comprises, wheel, shafts, gears then more bearings and a shaft with chains connecting to and rotating the front wheel assembly via attachments to the spud pan ring.
The ashpan could be glimpsed in the previous image. This I constructed in brass. It supports the firegrate and in the bottom is a non-scale opening' trap ' arranged such that it can be rapidly opened in an emergency as well as in normal use where it would be used for dropping the fire at the end of a run.
The parts shewn here finally assembled.

I was able to test the engine ( The part that sits on the boiler ) using an air supply. Later I satisfied myself that everything worked when I ran the completed Allchin, again on compressed air.



I enjoy the appearance of the various metals in the raw and will not be steaming up on coal

.............and not painting the model, although it will have a coat of instrument varnish.

More detail will be added to this page as time permits

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