Modelling the Kerr-Stuart Brazil Class 0-4-2ST locomotive in 16mm scale. - Keith Bucklitch



1. Tools.


Before you begin your boiler, you will need:


  • A blowtorch. - Most DIY shops sell these. The one you want has a burner head about 1" diameter. Buy several tins of propane/butane mix gas at the same time.
  • Silver-solder -This is available in 1.5 mm sticks. You should buy at least 5 sticks if this is your first boiler.
  • Flux - special flux is required for silver soldering. This is obtainable at the same source as your silver solder. This is a powder, some of which is mixed with water to a creamy paste and painted on the surfaces to be joined.
  • Citric Acid Crystals - this is used to pickle your boiler after soldering. It is obtainable from the Home Brew shop. Buy a 500 gram bag and dissolve about half of it in a 2 gallon bucket of water. Citric Acid is much safer (and easier to obtain) than Sulphuric Acid and works just as well, although you may have to leave things in it slightly longer.
  • Firebricks - from a builders merchant. Get 5 of these to make a hearth for soldering. Lay them in an open-sided box-shape to retain and reflect the heat on to the boiler.
  • Some material for making the flange plate formers. This can be steel, acrylic plastic or tufnol. A piece about 6 - 10 mm thick is required.
  • Tongs for handling the hot boiler. I found some barbecue tongs which were ideal for this.


2. Boiler Bushes


Before you begin the boiler work, make all the bushes you will need.


These should be turned from gun-metal or bronze. Do not use brass for boiler bushes. This is a simple turning job and should not need describing. You will need one bush with a 1/4 x 40 thread for the safety valve, four 3/16 x 40 bushes for regulator, blower, water glass and clack valve and a male bush for the pressure gauge take-off. You can also make the plugs for these at the same time so that when you are ready to test the boiler, the plugs will be already to hand. Additionally you should make the turret for the upper water gauge fitting.


3. Boiler Barrel.


The barrel of the boiler is made from 42 mm diameter copper tube. This is a standard size for heating systems, so chat up a friendly heating engineer for a few scrap lengths. The wall thickness is about 1 mm (18 gauge). The firetubes are copper pipe, 8 mm (5/16") and 11 mm (7/16") diameter. (Alternatively, you could use two pieces of 15 mm water pipe.) A length of 3 mm (1/8") pipe is used for the blower pipe. You will also need some 16 gauge (1.5 mm) copper sheet for the end plates.


Cut a piece of 42 mm copper tube 157 mm long. To get a perpendicular line all the way round the tube, take a piece of paper and wrap it around the tube so that the edge of the paper lines up. Cut as accurately to this as you can and file carefully to the line. Remove any burrs on the inside of the barrel. scribe a line around the barrel 3 mm from one end. This will be the front of the boiler. Scribe a line around the barrel 40 mm from the other end. Lay a piece of 12.5 mm brass angle along the length of the barrel and scribe a line along it. This will now be the top of the barrel. All your other marks must be made in relation to this. Drill a hole 29 mm from the firebox end on this line and open out to 12 mm diameter. You now need two lines along the barrel, at the 90 to the top line position at the firebox end as far as the circumferential line. You also need a line at the 180 position. Cut the barrel along this lower line, and up the circumferential line to the half-way line. Bend open these flaps to form the firebox walls. Remove any burrs and scour all the joint lines where soldering will take place with emery paper, wire wool or fibre-glass brush to thoroughly clean the copper. Later you will need to do some filing at this end to form a tongue to project through the backhead.



4. Front Tube Plate.



Make a flange plate from some suitable material. This should be turned to a disk 37 mm diameter. Slightly round off the corner of one side. The copper will be flanged over this corner. Mark the position of the various holes that will be made in the smokebox tubeplate and drill these 1.5 mm. You can then use the flanging former as a jig to drill pilot holes in the tubeplate later. Cut a 50 mm diameter disk of 1.5 mm copper plate and place the former centrally on this. Clamp in a vice and hammer the copper over the former. Turn the disk round and work your way around the circle. As you work, the copper will harden, and the hammer blows will start to ring. It is time to anneal the copper. Remove from the former, place the copper on a firebrick and heat to redness with the blow torch. Plunge this into cold water and you will find the copper is once again soft and malleable. Replace on the former and repeat the hammering technique, annealing each time the copper hardens. When you have an even flange all the way round, try the fit in the boiler barrel. With luck your tubeplate should be a fairly tight fit. It must not be loose otherwise you will be unable to get the solder to fill the gap. If the tubeplate will not enter the barrel at all, grip it in the chuck of the lathe using the inside jaws and lightly turn until it just enters the barrel. Use a slow speed and lubricate the copper with water, applied with a brush as you turn. Take care not to dig the tool into the copper. When the plate is a firm push fit in the barrel, replace the former and drill the pilot holes for the tubes. Open these out very gradually to the required size. Finally clean off any burrs and scour the edges where solder will be applied.


5. Firebox backhead/tubeplate.



Make the firebox backhead/tubeplate from 1.5 mm (16 gauge) copper sheet. Cut to size and mark the position of the tubes, bushes and the fold lines. A slot is cut for a tongue formed from the barrel to fit through as a stay. Drill the holes for the tubes and bushes. Fold to shape and try the fit within the barrel. File carefully until the backplate fits closely. The upper part of the barrel must now be filed away to form a tongue which fits into the slot in the backhead and a recess for the backhead to lie flush with the rear of the barrel. The backhead will butt up against the end of the barrel. Cut the girder stay and fold to shape. Cut lengths of copper tube for the firetubes and check their fit in the tubeplates. They should not be very loose. Ideally, the tubes should require very light turning so they just slide into the tubeplates and will stay in place without dropping out whilst being soldered. Cut the rear end of the tubes at an angle.


6. Girder Stay


Clean the lower surface of the girder stay and the top of the firebox crown sheet (the flat part of the backhead) with a fibreglass brush, and rivet the stay onto the flat area of the backhead with two 1/16" copper rivets. Anoint the joint with flux paste and place on your hearth. . Place the jar of flux powder within reach but away from the flame. Gently apply heat until the water has boiled away. Heat more strongly to redness. When hot enough pass the solder through the flame to warm it, dip it into the flux power and touch the solder to the joint. If hot enough the solder will melt into the joint. Continue to heat until the solder has run right under the stay and around the rivets. Pickle in the acid and clean up. Make sure there is a good ring of solder visible around the rivets. After the stay has been soldered in place clean up the barrel, assemble the backhead in the barrel, anoint the joints with flux paste and place on your hearth of firebricks. Apply heat gently to the barrel until the water in the flux has boiled away. Increase the heat playing the flame around the backhead/barrel joint. Now concentrate the heat at one point. When the metal glows red, touch the silver solder stick briefly in the flame, dip in the flux powder and apply to the joint. If the metal is hot enough, the solder will run into the joint. (The common trouble beginners have is not heating the metal sufficiently.) Slowly move the flame around the joint and the solder will follow the heat as long as the metal is hot enough. Apply more solder from time to time, but don't be excessive. The object is to fill the joint but not cover the metal. (The boiler will be pressure tested, not Hall-Marked!) When you have been round the joint there should be a thin ring of silver all round. Repeat at the tongue. Allow to cool to black then dunk the barrel in your bucket of Citric Acid. Leave for 5 minutes then examine the joint carefully. You should have a thin line of silver all the way around the joints. If you have obvious gaps you will have to pay some attention to these at the next heating. Large gaps will need to be plugged either with small slivers of copper hammered in the gap, or by closing up the gap by gentle application of a hammer. The metal will be well annealed and should be quite easy to shape at this point. In fact too enthusiastic hammering may produce an unwanted dent somewhere in the barrel.




7. Clean the holes in the tubeplate and the smokebox end of the barrel. Cut the firetubes to length. Note that these are chamfered at the firebox end. Clean the ends with emery paper or fibreglass brush and insert into the firebox tubeplate. Insert the smokebox tubeplate into the barrel and thread the firetubes through it. Insert the blower tube. (Mark any suspect joints from the previous heating with a pencil.) Insert the boiler bushes into the appropriate holes. Anoint ALL the joints with flux paste. Stand the barrel on its front end and apply heat as before. When hot enough, apply solder around the tube joints. Turn the barrel end for end, apply heat to the front tubeplate and barrel and again apply the solder. Make sure that you have soldered all the way around the tubeplate and the tubes. Pickle as before and examine all the joints again.





8. Make the wrapper (drawing not shown) for the steam space from a length of 42 mm tube and the front crescent from 16 gauge copper. Carefully fit the wrapper and crescent to the barrel. Clean around the joints again, insert the bushes into the various holes (including the top and bottom water gauge feeds), apply flux and heat as before. (NB. When measuring for the bottom water gauge feed, the length should be sufficient to bring this in-line with the bush for the top fitting) Pickle again and examine all the joints.


9. Hydraulic testing of boiler.


Now it is time to test the boiler. You will need some blanking plugs made to fit all the boiler bushes. Wrap some PTFE tape around these and screw into the holes. Before doing a hydraulic test, I usually do an air test for leaks first. Having plugged all the holes, drop the barrel into a bucket of almost boiling water. Observe carefully for any bubbles from the joints. If any are found (not unusual) mark the position of these with a pencil. Scrape and thoroughly clean the metal around these leaks. Sometimes the cause is dirt in the joint, sometimes insufficient heat. Apply flux paste to all joints, reheat and apply extra solder at the leak points. After pickling empty the boiler and repeat the hot water test. When no further leaks are detected, we can perform the hydraulic test.


You will need a large diameter, accurate pressure gauge to test your boiler and a hand pump to pump water into it. The Association has a number of boiler testing kits and Dave Shotter amongst others will often perform this task at various shows. Having acquired your pump and gauge, connect them to the boiler. Fill the boiler with water until all the air is expelled. Plug the final hole. Slowly, pump more water into the boiler and watch the pressure gauge. Initially raise the pressure to about 25 psi. Examine the boiler for any leaks. These are most likely from around the plugs. Apply more PTFE tape to the plug and raise the pressure again. The operating safety valve pressure for the loco will be 40 psi. so in theory the boiler only needs to be tested to 80 psi. However, I usually test my boilers to 100 psi to be really sure. Raise the pressure slowly to 100 psi. Examine the boiler carefully for any leaks. If the pressure falls slowly, the cause is often a leaky pump. Pump more water in to maintain the pressure. Hold the pressure for 5 minutes, examine the boiler carefully, especially inside the firebox and backhead. If no leaks are found you can be assured that your boiler is safe. Make a note of the date and test pressure so that you can repeat the test in two years time. Better still take the boiler to one of the shows and get it tested and a proper boiler certificate issued. (If you join your local model engineers society they will test the boiler and issue a certificate for you.)


Next Firebox.

If you have any questions, or comments or find any errors in these notes please contact me by email. Email Keith Bucklitch.

Copyright © 1998, Keith Bucklitch
Last Edited - November 1998