Back to boiler making!

The next stage on the boiler was to fit the firebox and tube asembly into the outer barrel. The firebox was inserted into the barrel and the tube ends eased through the holes in the smokebox tubeplate using a bit of rod inside the tubes to line them up with the holes. The front section of the foundation ring was then cut and fitted into place. When I was happy that the firebox was central in the barrel, a couple of 1/16" rivets were put through the section to hold the firebox in place and then the crown stays silver soldered to the inside of the barrel (These were a good close fit to the barrel so it was not necessary to fasten these at all). To do this I rested the barrel upside down so that I could get to the underside with the torch, fluxed the joints well and laid a length of Easyflo along the side of each crown stay. The barrel was then heated until the solder melted and flowed between the stays and the inside of the barrel.

The two long side sections of the foundation ring were then cut and fitted and held with a few rivets. The front and sides of the foundation ring were then soldered with Easyflo. At this stage I did not intend to fit the backhead. This will be done last after all the side stays are fitted along with the rear section of the foundation ring. The last silver soldering job for the moment was to solder the tubes into the smokebox tubeplate, again using Easyflo. A ring of solder was dropped over each tube, the whole lot fluxed, and then heated until the solder flowed. So far, so good!

Front and sides of foundation ring fitted and silver soldered

Tubes silver soldered into tubeplate

Some thought was then given to the side stays. I had already decided to use 1/8" copper rivets for the stays which will be threaded 5BA and screwed through both the inner and outer wrappers. The ends inside the firebox will have brass nuts fitted and caulked with high melting point soft solder (Comsol). I think it would be virtually impossible to silver solder these inside the firebox without the use of Oxy-Acetylene equipment which I do not have access to. I had originally planned to soft solder the outer heads of the rivets as well but I reckon it will be fairly easy to silver solder these instead judging by the ease with which I managed the foundation ring.

I had intended to use a stay spacing of ¾" which is what most boiler designers (LBSC, Martin Evans, Don Collin/Paul Wiese) seem to use for 5BA copper stays but I began to have doubts about this after reading various books, articles etc. In Martin's book on boilers he gives a formula for calculating the stay spacing based on the diameter of the stay and when I used this I got a spacing of only ¼"!! Looking at the graphs on Jim Ewins website re boiler design suggests a spacing of ½". I came across a spreadsheet on an Amercican website for calculating boiler specs and putting the relevant figures in this and using a spacing of ¾" gave a safe working pressure of only 80psi for a safety factor of 5. In the end I have decided to reduce the stay spacing to 5/8". Using the spreadsheet again gives a safe working pressure of 112psi for a safety factor of 5. As I will probably only run the boiler at 90psi this gives a safety factor of over 6. This will mean fitting more stays but a bonus is a slight increase in heating surface due to the extra stays!

The stay positions were measured from a CAD drawing and marked out on the outer wrapper and a countersunk made at each position using a centre drill in a cordless drill. I did not think it wise to try centre punching the positions as this would have no doubt distorted the wrapper. Incidently, due to the different curvatures of the inner and outer wrappers, the stay spacing will vary from top to bottom of the wrapper. If you make all the spacings on the outer wrapper 5/8" the spacings on the inner wrapper may well be more than this. It's necessary to alter the positions of the stays so that no spacing on the inner or outer wrapper exceeds the required figure. This problem does not occur on a rectangular firebox of course.

The next problem was threading the rivets. As they needed to be threaded right up to the head they could only be held by the head which was not easy. Fortunately it does not matter if the heads get a bit chewed up as they are going to be soldered anyway. I found the easiest way was to grip the head in a drill chuck in the lathe and thread the shanks with a tailstock die holder. To make it easy to line up the rivets in the chuck I drilled a clearance hole in the end of a piece of bar held in the tailstock chuck. The rivets were placed in the bar and the bar slid up to the lathe chuck until the head entered the jaws which were then tightened to grip the rivet.

Rivet inserted in bar prior to gripping in the chuck

Threading rivet with tailstock die holder

After threading the shanks the heads of the rivets were slotted with a saw so that they could be screwed tightly into the boiler.

Finished stays

The boiler shell was then drilled and tapped for the stays one row at a time to keep any distortion to a minimum. The drilling was done in the micro-mill with suitable packing under the boiler shell so that the holes went through the shell at the correct angles. Lubricant was used when tapping the holes to ensure nice clean threads as copper can be nasty stuff to tap and tears easily. The outer holes were countersunk slightly to ensure the stays went in as far as possible and the heads touched the wrapper without a gap.

Top row of stays fitted


All side stays and the throatplate stays fitted

The bottom stay on each side at the front of the boiler had to be treated slightly differently as these are below the tops of the frames and the boiler will only just fit between them. Consequently these two stays cannot stick out from the boiler shell and must be flush on the outside. I got around this problem by countersinking the boiler shell deeply and turning the head of the stay to suit. This gave a good contact area for the silver solder and should be strong enough. The head of the stay will be filed flush with the shell after soldering. The actual area these stays support is fairly small so there should not be that much load on them anyway.

All the stays fitted were then silver soldered to the outer shell using Easyflo. I managed to do these all in one go by tackling one side first and then turning the boiler over to do the other side.

All that remained now was to fit the backhead and the last piece of the foundation ring. Fortunately the backhead was still a good fit inside the shell and the firehole still lined up properly! The section of foundation ring was cut and fitted in place with a couple of 1/16" rivets. After checking that the backhead was square and vertical to the rest of the boiler, it was secured by drilling and tapping a few 7BA holes through the shell and the backhead flange and screwing in bits of threaded copper rivet which were then cut off slightly proud of the shell. The shell was then tapped into close contact with the backhead all the way around. Last job was to peen over the firehole ring to secure it to the backhead. I didn't do this too tightly as I wanted to leave a small gap for the solder to penetrate the joint. A couple of small gaps in the foundation ring were filled by tapping in slivers of copper and then the whole lot was silver soldered with Easyflo. After pickling the boiler the joints all looked sound but if any slight leaks are found on the hydraulic test I'll seal them with Comsol as I don't particularly want to have to heat the boiler up to silver soldering temperatures again.

Backhead fitted and silver soldered

All that remains now is to fit the brass nuts on the firebox ends of the stays and caulk them with Comsol and tap the threads in the bushes all the way through. I'll also do a bit of cleaning up as there's a few blobs here and there where I've been a bit generous with the solder. Then it's testing time!


I spent about three hours this morning fitting all the brass nuts onto the firebox ends of the stays. I've now got extremely sore finger ends! To say it was a bit of a fiddly job would be an understatement!! It's a good job I've got reasonably long thin fingers as there is not a lot of room to get in at the stay ends. Fortunately I've also got a 5BA ring spanner which helped to tighten the nuts up after actually getting them onto the stays. Looking back, I think it would have been easier to fit the nuts at the same time as the stays were fitted.

Brass nuts fitted to stays inside the firebox

The nuts were then caulked with Comsol using Bakers fluid as flux. This job still takes a fair amount of heat as the solder melts at 300°C. Rather than use the gas torch I laid the boiler on its side across the largest burner on the gas cooker and relied on the heat conducted along the stays to melt the solder. Plenty of Bakers fluid was sloshed all over and the head of each stay and its nut were given a good dollop of solder. After all the stays had been annointed with solder more Bakers fluid was sloshed over the joints just to make sure! The boiler was then turned over and the other side done. After a good clean up (boiler and cooker!) it looked as though all the nuts had been well and truly sealed. Please note that under no circumstances should ordinary plumbers solder or 60/40 electrical solder be used for this purpose as the melting point is not high enough!

Stays after caulking

The boiler bushes were then tapped all the way through ensuring the threads were square to the boiler surfaces. I then realised that I needed to fit the blower stay and the solid longitudinal stay to the boiler before I could think about doing a hydraulic test so those will be the next job. There is some doubt as to whether these stays actually do anything at all in small boilers and Martin Evans often did not fit the solid stays to his later boilers, relying on large diameter bushes on the backhead and the smokebox tubeplate to stiffen them. Looking at Helen's boiler I doubt if they will do much as the unsupported area on both backhead and tubeplate is very small and unlikely to bulge. We still need a blower stay though to get the steam from the blower valve to the smokebox!

Both stays will screw into the backhead and tubeplate. The blower stay will be 3/16" dia. copper tube silver soldered into the end of the blower valve and will have a threaded sleeve with a lock nut at the smokebox end. The solid stay could also be 3/16" copper but I'll use bronze as I've got some! This will have a threaded cap silver soldered on at the backhead end and another threaded sleeve at the smokebox end. I've taken this idea from the Flying Scotsman boiler design by Derek and Paul. Interestingly, they suggest that stays silver soldered in position, as recommended by some designers, may actually be useless. Due to differential expansion between the boiler and the stays during the soldering process the stays probably expand lengthways less than the boiler and after cooling they may sag when the boiler contracts back to it's normal length, in which case they will obviously not take any load at all!


The body for the blower valve was made first and is a simple turning job from 5/16" hexagaonal bronze bar with a 3/16" dia. spigot silver soldered in for the steam pipe from the manifold. It's quite a pleasant change making little fittings like this as they take no time at all to make. The valve itself and the gland etc will be made later. Incidentally, I think it now a stipulation of the new boiler regulations that all valve spindles must be captive i.e. they cannot be unscrewed so far that they can come out of the body with possible serious injury from scalding steam. At the moment I think this only applies to new boilers, not to existing ones.This is fairly easy to do by making the actual valve spindle smaller in diameter than the screw thread so that the screwed part of the valve cannot come through the valve gland nut.

The screwed ends of the stays at the smokebox end are simply threaded bits of 1/4" dia. bronze bar, the blower one being drilled through and countersunk on the outside to take the blower pipe union.

Blower stay and solid stay

The stays are fitted by screwing the smokebox end all the way through the backhead and then pushing them through the boiler until they can be screwed into the smokebox bushes. After a few turns the threads on the backhead ends start to engage as well. I used commercial copper washers under the blower valve and the head of the solid stay to seal the joints and also coated them with Foliac jointing compound. Actually the copper washers were much too big in outside diameter so I turned them down in the lathe to the same diameter as the backhead bushes. Washers were also used at the tubeplate end under the locknuts and Foliac again applied to the threads. In the Flying Scotsman series it suggests caulking the stay joints with Comsol but I'll see how I get on with the way I've done it.

I now need some blanking plugs to seal all the various holes and bushes in the boiler ready for the hydraulic test. I turned up the blanks for the inner dome and the regulator fitting on the backhead from bronze bar but I've not drilled the regulator fitting for the operating rod as that would be another hole to fill in! I'll do the same for the steam pipe connection on the smokebox tubeplate. The dome and the other fittings can then be screwed into place to seal those holes. The various screwed bushes can be sealed with simple plugs made from hexagonal brass. I've already made some screwed blanks for the safety valve bodies so they can be utilised as well.

Backhead showing ends of blower stay and solid stay and the regulator boss blank

Smokebox end of stays and blower pipe union


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