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JMD Belpaire boiler assembly

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I finally got round to writing the instructions for the Y Boiler 650 Class I thought it would be useful to prepare a tutorial for assembling a Belpaire firebox as I will use the same concepts for the X & Z boilers fitted to the Midland & Southern Standard goods and a number of other classes. The same principals can be applied for scratchbuilding a boiler and firebox.

The boiler and fire box wrapper are half etched and easily bruised so a level of care is required in assembly.

Stage 1.


Pre-formed boiler wrapper, boiler front and back plates and firebox former/assembly jig.


Firebox front and back formers with assembly jig bent to shape the vertical half etched line in parts 14 is to assist in aligning the firebox wrapper.


Firebox jig assembled captive bolt fixed to boiler front plate, captive nuts fitted to back plate. This stage of the soldering is carried out with a medium temperature sheetmetal solder (179°) to reduce the risk of the soldered joints failing at a later stage of assembly using a lower temperature detailing/general purpose solder (145°). I oiled the threads of the backplate bolts with a light oil before soldering the nuts to the backplate, this avoids soldering everything solid.


Boiler front plate tack soldered in position with 145° solder to allow adjustment before finally soldering the plate in position. The firebox former is soldered with 179° solder.


Boiler back plate soldered in place with 145° solder, don't loosen the bolts until the back plate is soldered in place!


Firebox wrapper there are raised alignment marks on the front and rear of the wrapper and rear of the boiler barrel. Bolts loosened on backplate.


Firebox wrapper tack soldered to the front plate to check alignment, there is a slight mis-alignment I will align and tack the wrapper to the back plate before re-aligning the wrapper.


Edited by Mayner
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Stage 2


Wrapper tacked to rear plate, re-aligned at front plate and tacked in place.


Wrapper soldered to front and rear plates, keeping well clear of curves! I basically applied firm hand pressure to the jig while soldering (in and out quickly!)


Forming the curve on one side by lightly pressing down on the jig, the piece of wood is to stop the firebox tipping over while I was taking the picture!



Firebox assembly jig 2! I basically clamp the sides of the firebox between two pieces of strip wood using hand pressure while running a light fillet of solder around the outside of the seam using 145° solder.

I use a mild phosphoric flux based on a dilute commercial rust treatment similar to Ranex Rustbuster, not sure of a similar product is available in Europe, soldering was with a 25watt Antex with a medium chisel bit.



Firebox formed to shape, the boiler and firebox are designed to bolt together.


Spacing jig removed from firebox.


Solder fillets formed all round between wrapper and front and back plates.


Temporary legs removed from front plate half etched overlay to be soldered in place, front plate pre-tinned.



I used a toolmakers clamps to ensure that the firebox did not disintegrate while soldering the overlay in position, the bolts are oiled and used to assist alignment during soldering.

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Stage 3.


Boiler barrel and outer firebox awaiting assembly.


Bolting firebox and boiler together. I originally used captive bolts in the boiler backplate but revised the assembly as it was difficult to tighten the nuts without causing damage to the wrapper.




Assembled boiler awaiting a loco! I form the curved fillets at the front corners of the firebox by gentle filing and smoothing using wet and dry paper.


View inside firebox the notch in the back plate is to provide space for running a power feed to the motor.


I slipped up by getting the hole for the handrail knob in the side of the firebox wrong. In practice it is extremely difficult to get pre-formed handrail holes in two components to line up however accurate the design or the level of care taken in the assembly. Its simpler and quicker to plug the existing hole with solder from inside the firebox and drill the holes in the firebox when the boiler and firebox are correctly lined up and assembled.

This boiler is ear marked for 653 the first of the class to be rebuilt with a Belpaire boiler as first re-built 653 was something of an oddity retaining solid valences with a short GSWR cab roof and sidesheets similar to the 52 Class and older GSWR passenger locos, but that's another days/6-12 months work.




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2 hours ago, KMCE said:

Superb work John. 

I particularly like the Firebox jig as a means of holding that section solid whilst applying the wrapper - very clever.



I basically borrowed the idea from Brassmasters (Martin Finney's) GW loco kits  using a fold up spacer rather than threaded rod to keep the profiles at the correct distance.

I allowed the bare minimum for etch factor and fitting tolerance in the spacing jig and considerable care and attention is required in the assembly to avoid mis-aligning or damaging the firebox wrapper. 

I  prepared the tutorial in order to finalise & support the instructions for assembling the kit  as the fiirebox assembly is a lot more complex that for the round topped versions of the class.


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  • 2 years later...
On 21/7/2022 at 9:39 AM, Horsetan said:

Continuing to wait this end for the Z boiler conversion for the J15 - hopefully it'll happen one day 🤔

The Z Boiler J15 conversion is on this years to-do list, its basically a matter of updating the CAD work to incorporate improvements identified as a result of the test build from the original artwork.


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