Large Prairie 4114

This is a hybrid, having an old Airfix body and a Martin Finney chassis. It is running on Alan Gibson wheels with Mashima motor and High Level gearbox.

Full building details below.....



The Body

I made a list of those things on the body that I didn’t think I could live with and the replacement of which weren’t going to cost me a fortune. I was hoping, where possible, to keep the cost to a minimum and only the motor, gearbox, wheels and number plates would be purchased specifically for this project. The rest would either be fabricated or dug out of my ‘loco bits’ box.


The most obvious were the steam pipes that were moulded as a solid block with the footplate and smoke box. They would have to go.

The moulded handrails would have to go along with oversized wire handrail to the boiler/smokebox.

The moulded lamp irons on the bunker would have to go. There were none on the footplate.

Similarly, the steps on the front of the tanks were too crude a representation to be worth keeping.

There was no lamp recess in the rear bunker.

The footplate and steps were too thick.

The front of the smokebox was a disaster.

I had some better whistles in my oddments box, so the Airfix ones could go.

I decided I could live with the chimney (quite good) and the safety valve bonnet (slightly over size but messy to replace with a casting).

Similarly, the tank top fittings could stay, as could the steps on the rear of the bunker. Other areas would benefit from extra detailing. At this point, I was still going to keep the buffer housings, but subsequently decided that they had to go.

computerThe steam pipes were carved away with a scalpel (10A blade for the serious demolition, finishing off with a No.10). A No.10 blade was also used on the handrails, steps and lamp irons, finishing off with very fine ‘wet and dry’ paper. The footplate was thinned down by scraping the underside, being careful not to damage the valances. The top edges of the tank bunker were treated in a similar manner. More brutal treatment was applied to the rear of the steps below the footplate. The front of the smokebox was carefully levered off and discarded. Finally the section comprising the rear of the cab, coal and bunker fender was removed and set aside for partial re-use. Happily, Airfix used a fairly mild adhesive when assembling the body components (or maybe they had deteriorated with age), so it came apart fairly easily.

It was now time to take a closer look at the fit between chassis and body. Firstly the pillar that takes the mounting screw on the Airfix chassis needs removing, side cutters initially, finished off with the scalpel. A couple of grooves in the rear of the buffer beams took care of the latter being over thick. Due to the thickness of the plastic footplate as compared to that on an etched kit, the body sat rather too high at the front. This was partially cured by scraping away at the underside of the footplate under the smoke box, but would also require surgery on the replacement chassis in due course. The over thick valances clashed with both the top of the cylinder casings and with the motion brackets. Local thinning of the valances cured this. Knowing that everything would now fit, I was able to start putting stuff back on the body.

First, there was still some serious work to do on the bunker. The rear of the cab and the bunker fender were each separated from the coal with a piercing saw before being tidied up. A slot was then cut in the rear of the bunker for the lamp recess. The enclosure for this was fabricated from 0.5mm thick styrene curved to match the shape of the fender to which it was then bonded. I used black styrene for most of the work on the body; it is softer and more malleable than white, but has the disadvantage of not being able to mark up clearly. I also found that styrene bonded well to the plastic rodsbody using butanone solvent. Once everything had hardened off, the whole assembly was thinned down somewhat. Prior to re-fixing the rear of the cab, the rear edge of the roof was reduced to a respectable thickness, again by scraping away at the underside. Recesses were also made in the cab sides to allow the back to be recessed into them as per the prototype. Sadly this could not be done for the cab front, but a thin styrene etching was added to the edge of the roof as a compromise. The rear of the cab was extended down to footplate level with styrene prior to fixing and, at the same time, tool boxes and basic internal detailing were added. Once the back had been secured in place, a false floor to take the coal was fitted with its top level with the cut out for the lamp recess. The modified fender was then secured on top of this and to the edges of the lamp recess. The joints were then smoothed out with Milliput filler and a thin strip of styrene applied and shaped to replicate the beading around the rest of the bunker. The mounting slots for the Airfix chassis were filled with styrene and finished off with Milliput. The bunker was now filled with lead, secured in place with epoxy, before fitting a 1.0mm styrene bottom. This was left reasonably high to allow a mounting plate for the chassis to be fitted later.

Prior to fitting new steam pipes, the gaps in the footplate and smokebox saddle were filled with styrene and finished off with filler. Once filed and sanded to shape, new bolt heads were applied to the saddle to replace the small moulded pips on the model. I used white styrene for these as it was easier to see! I also mockedcross up representations of snifting valves in styrene and secured these to the saddle. The steam pipe flanges (black styrene) on the smoke box were fixed first, followed by the steam pipes themselves. These were formed from 3mm OD brass tube, carefully cut to length and shape and secured with epoxy. Getting these correct was a matter of trial and error and a couple of rejects were discarded. 0.25 styrene overlays were fixed to the top of the motion plates to make them look a little more like the real thing. Oil pots and bolt heads were added to the motion bearing covers. While working in this area I made up some new steps for the front of the tanks from 0.25mm styrene.

I also decided to replace the boiler stays as they didn’t look right, but at this point didn’t know why; this meant cutting the old ones to get them out. 0.25mm styrene plates were secured to the smokebox, centred on the existing holes. The struts were formed from 0.7mm hard brass wire, again a case of trial and error to get the length and angles of bends correct. Styrene plates were then slotted around the foot of the struts where they entered the footplate. Later on, I decided the struts still didn’t look quite right, even with their end plates, and it was a while before the penny dropped. I should have filled the original mounting holes and drilled new ones in the correct place. Next time I will know better!

crossI made a new smoke box front and door from styrene. I have an Olfa cutter that makes cutting circles a lot easier. The circular blank for the door was mounted on a stud (using the hole for the dart) and scraped, filed and sanded  to shape in the chuck of a small electric drill clamped in a vice. This had to be done very carefully, as black styrene has a fairly low melting point and tears easily. A smokebox dart came out of my bits box and the hinges were made up from styrene and brass wire.

An ATC cable and clips were made up from thin brass wire and brass shim. On the prototype, these were either hung just below the valance or on the face of it. I chose the latter as it makes it a lot less vulnerable to damage.

The moulded  handrails were replaced with hard brass wire; 0.33mm for the smaller handrails and 0.45mm for those with knobs. Additional handrails were fixed to the LH cab side and roof and on the boiler band forward of the side tanks. A replacement handrail and knobs for the boiler/smokebox was also made up with 0.45mm brass wire.

Lamp irons came from my bits box, were folded to shape, drilled for 0.33mm brass fixing pins, soldered, then fixed with superglue into pre-drilled holes. The cab front was drilled for the replacement whistles (again from my bits box) which were secured with epoxy.

Steps for the LH side of the bunker (missing on the original model) were made up from 0.25mm styrene. These are quite easy to make. Cut one reasonable length strip the depth of the step and one the height of the side pieces. Cut the latter into the lengths required (the depth of the step). Mark out the wider strip to the width of the steps, leaving gaps between them. Secure the side pieces on top of the wider strip in the marked locations with butanone. Leave to harden before forming the chamfer on the side pieces. Cut the individual steps from the strip. Voilà.

By now I had decided to fit complete sprung buffer units (Alan Gibson from the bits box), so the plastic ones were removed with a piercing saw, the buffers beams cleaned up and drilled out to the appropriate size. Only the housings, secured with epoxy, were fitted at this stage. The buffer beams were also drilled for screwed couplings (Exactoscale), again, only the hooks were fitted for now.


Mounting vacuum and heating pipes securely can be a problem, which is probably why I sometimes find the odd one in the bottom of my stock box. The overcome this problem, I built up mounting blocks in styrene on the underside of the front footplate and on the base of the bunker. These have to be narrow enough to just fit inside the mainframes. Short lengths of PCB sleeper were grooved to provide three electrically separated surfaces (one each for the two pipes and the centre one for the AJ coupling), which were then tinned. The position of the AJ coupling will determine the exact height of the blocks. The PCB’s were then fixed with epoxy. The pipes were formed from hard brass wire and wound with fuse wire to represent the flexible section. This is best done prior to trimming the wire to length as it can then be held in the vice for the winding process. The fuse wire is secured with a blob of solder that can be filed to represent an end fitting. Mounting brackets for the buffer beam were made up from brass shim. The finished pipes were then soldered into their respective positions. This is where the reason for the isolation of the three sections becomes evident, as the first pipe won’t move when you solder on the second one and neither should move when the AJ is soldered in place. Yes I do have rigid AJ’s on my locos and rely on the movement on wagon/coach couplings to enable satisfactory uncoupling/coupling.

The last bit of work (prior to painting) was to add more weight. I use offcuts from lead flashings gleaned from building sites when I worked in the construction industry. I made a ‘swiss roll’ to go in the boiler, substituting epoxy for jam, and glued layers of lead in the forward sections of the side tanks, squeezing in as much as I could to get the centre of gravity over the centre drivers. The finished loco weighs in at 250gms.

After a thorough clean, rinse and drying off, the body was sprayed with a relatively light coat of Halfords grey primer. Once I was happy with this, the whole body was sprayed with two coats of Railmatch ‘coal black’. When this was suitably dry, the boiler and tanks were masked off and the smokebox and footplate were given one coat comprising a mix of coal black and a little matt black. Transfers are Pressfix from HMRS. Cabside number plates from 247 Developments were fixed with a dab of black paint. Smoke box number and shed plate were fixed with epoxy. Weathering was left for later.



The Chassis

The first task was to decide on the gearbox and motor configuration. Having measured up both body and chassis, I transferred this information to Autocad and started playing with the various options. My original chassis has a Portesap motor/gearbox driving the rear axle but I have moved away from Portescap and now favour a Mashima/High Level combination. The Finney chassis is designed to be capable of being fully compensated, the front axle rocking on a fixed pivot with the two other axles linked by a pair of compensation beams. There have been numerous discussions in the past on how well (or not) suspension systems work when one compensated axle is subjected to torque from the motor. The science is beyond me, but I do like the idea of driving a rigid axle.  I was therefore going to drive the rear axle and compensate the other two. Then I had an interesting thought. One of the other problems associated with P4 locos, is the lack of clearance between the back of the cross-head and the leading crankpin. This is exacerbated by the small amount of lateral play required to allow the leading compensated axle to rock. Why not drive a fixed leading axle and kill two birds with one stone. Fortunately, large tank engines have plenty of space to hide gearboxes and motors. This is a reverse of the system used by Malcolm Mitchell on his Mogul kit, except that has a single central compensation beam, rather than a crosspair bearing on the axle boxes. Once I had made the decision to go down this route, it was no problem to make up one hefty suspension beam from the two provided on the etch. Back on the computer, I found that I could easily fit a Loadhauler + and a Mashima 1430 gearbox into the space available. I rang Chris at High Level and discussed my proposals with him. His only concern was that this combination would not give particularly fast running as the 1430 is quite a slow running motor. That was not a problem for me so having settled on a 1:60 gearbox, the order was placed.

The one drawback with this arrangement was that the gearbox would clash with the frame spacer between the leading and middle axles. This was easily solved by reversing the spacer and reducing the length of its lower leg while leaving enough of the tag to locate it in the wrong end of the slot. There were two more changes to make to the chassis as designed. Firstly, to overcome the lack of clearance on the radial axle, I would use the EM rather than the P4 rear frame spacer. Secondly, the rear leg of this frame spacer would clash with my pipe/coupling mounting block on the body. I had foreseen this and decided that it was not a problem as this fourth leg of the spacer is only there to provide support for a pair of cosmetic (in model terms) supports to the rear bunker that are not normally visible. This leg and its attached supports could therefore be removed.

Having made the necessary changes to the spacers, the basic chassis was assembled using my Avonside Works Chassis Pro jig. How did I ever manage without it! The new chassis could now be paired up with the body to check for fit. In fact it fitted very well, the extra waisting at the back slightly reducing the overall length (this would however have repercussions later). However, I still had to deal with the problem generated by the over thick  footplate. I reckoned I had to take between 0.25 and 0.5mm off the top of the front section of the main frame. The frames were filed down a little bit at a time until the body sat nice and level on the chassis. This did however leave the cylinders sitting too high, so the locating slots in the frames had to be extended accordingly using a piercing saw. The cylinders are now a scale 1” too low, but I don’t think it’s too obvious. The next job was to make and locate fixing brackets on the body. The front fixing bolt passes through a frame spacer and the cylinder assembly (thereby holding the cylinders in place). The top of the cylinder assembly is recessed so the body needed packing out at this point with styrene, remembering to leave a hole for the captive nut. A plate was cut from nickel silver sheet and an oversize hole drilled for a 10BA bolt. A 10BA nut was soldered to the back of the plate before it was fixed in place with epoxy, its exact location having been carefully marked beforehand and double checked with the chassis in place. The similar procedure was carried out at the rear (see pic above).


The cylinders, connecting rods and coupling rods were now assembled as designed. I measured the distance between the rear faces of the crossheads and compared it with the distance over the leading crankpins (back to back + 2 x wheel thickness including front boss + 2 x depth of crank pin projection). Oops, the former was 0.35mm less than the latter, not good news. Did I have this problem last time? Are Alan Gibson wheels slightly thicker than Ultrascales? I know in the past I have resorted to moving the slide bars out a little, but it was a bit late for that now and I don’t like thinning down the ends of the coupling rods. In the end, I reduced the thickness of the front bosses on the front drivers, they are, after all, pretty much hidden from view. The centre drivers raised another problem. I had assembled the chassis without removing the cylinderscusp on the frame spacers (more surface area for the soldered joint?) knowing that the chassis was designed with a detailed overlay which I didn’t have (or so I thought), so I would be OK on overall width. I had forgotten that the bearings project beyond the outer face of the basic chassis. These therefore needed filing down to give me sufficient lateral movement on the centre axle. This didn’t solve the problem completely so I took a little of the rear wheel bosses as well. I didn’t need so much play on the rear axle, so the bosses were left alone. Too much play there just makes matters worse on the radial axle. After fitting Ultrascale crankpins, the wheels, axles, bearings, etc. were assembled using my GW Models quartering jig with the leading axle in-situ in the chassis. Needless to say I forgot to adjust the length of this axle to compensate for the reduced bosses, but that was soon rectified. Everything looked OK, there was no play on the leading axle and the wheels spun freely, so I popped the radial axle in and headed to the layout for a test run. I set the chassis down on a private siding, accessed via a 900mm radius bend, that would normally only be used by short wheelbase locos. It just scraped around, but then ran freely round the access to the shed complex which is about 1100mm radius. Excellent. I was now able to check my compensation beam which was not yet permanently fixed in place. With the chassis sitting level I found that I could get a thin piece of card between one axle and the beam so a suitable thickness of nickel silver was soldered under one end of the beam. The beam pivot could now be soldered in place and the ends filed flush with the frame.

The coupling rods, connecting rods and cylinder assemblies were now fitted. Everything still ran smoothly, testament to the accuracy achieved by using jigs for assembly and wheel quartering.

The pony truck was now assembled as designed, a few missing bits of bracing were fabricated from brass wire and nickel silver offcuts. The only real modification was to the springing. A second hole was drilled in the front top cross bogiemember to take an extended spring wire. The combination of this, together with bending the spring wire downwards, put more load on the bogie as well as giving greater lateral control. The pony truck was then secured on its pivot and the chassis was tested again. Moving to the rear of the chassis, twin spring wires were fitted to bear on the radial axle. Being relatively short, these are quite stiff and keep the wheels in firm contact with the track.

With everything working OK, cylinder wrappers were fabricated from thin brass sheet and, after rolling to shape, were soldered in place. Shortly after these were completed, I discovered that I actually had the body etches that contained the missing chassis parts! The first to be used were the assemblies for the cylinder drain cocks, the drain cocks themselves were fabricated from brass wire and tube. The parts that I had just made for the pony truck and the cylinder wrappers were also on the etch!

The chassis was now stripped for painting. At this point the springs (which also retain the horn blocks on the compensated axles) were soldered in place. The front sand pipes were formed from 0.4mm brass wire and soldered in place. The rear sandboxes were scratch built from black styrene and secured in pace with epoxy. The brake cross shaft, levers, etc. were soldered in place as was the operating gear for the rear sandboxes. PCB strips for power collection were cut to length, tinned and secured in place on the main frames with epoxy. A short length of square brass tube was soldered across the chassis as part of the retaining system for the motor. I used gun blue on the coupling and con rods. The wheels had the balance weights attached before also getting a carefully applied coat of gun blue.

motorThe chassis, pony truck and cylinder assembly were now primed and painted, after first masking horn blocks, bearings and slide bars. The wheels had a quick waft with the airbrush at the same time, excess paint being easily removed with a light application of white spirit on a cotton bud. Two back scratchers per wheel were soldered to the pcb strips on the main frames and wired up. The backscratchers are made from phosphor bronze strip with a tiny bit of fine brass wire (I have heard that some people use gold) soldered on one end to make contact with the back of the wheel. I opted for two as this balances the load on the wheels, the braking effect is not too high and I knew I had a motor with plenty of ‘grunt’.

The chassis was now re-assembled, this time with the motor and gearbox on the leading axle. The unpowered testing procedure was carried out pretty much as before and no major problems were encountered. The motor was connected up, the grub screw on the final drive was tightened onto the axle, the chassis was placed on the track, power applied and off she went. Happy with that, the retaining strap over the motor was soldered in place and the chassis was mated with its body. Duly weighted down, it now underwent rigorous testing round tight curves and backwards and forwards through turnouts. As promised, no high speed running but excellent low speed running.

us chasTesting over, the body was removed again. Having found the missing etch, I was now able to assemble the brake gear without resorting to scratch building. This all went together as per the instructions, but the shortening of the chassis mentioned earlier meant that the pull rods were now too long. However this was not a problem as they were only going to sit on top of the cross shaft anyway. A little trick that I employ to get the brakes hanging the desired distance from the chassis is to thread a short length of small diameter wire insulation onto the hanger wires. As I had chemically blackened all the brake gear beforehand, the brake hangers are held in place with a very small spot of epoxy.

injectorThe last two assembly jobs were to fit the rear sand pipes, formed from 0.4mm brass wire, and the fabrication of the injectors that sit behind the rear steps. Representations of these were made up from various sizes of brass wire and tube and, once soldered and cleaned up, were secured in place with epoxy.

The body and cylinder assemblies now went back to the spray booth for a little light weathering.

cabFinally, I got around to doing something about the inside of the cab. This was assembled as a single removable unit that is only held in place by the chassis. The floor, back head and wheel splashers were fabricated from black styrene, The reversing lever was filed down from a length of 1.0mm brass rod and detailed with brass shim and off cuts. The brake standard is 1.0mm fine bore brass tube drilled to receive the 0.3mm brass wire that forms the handle. The loco crew are from Dart Castings.





ind stock constr