The backhead carries one blowdown valve and two clacks for the water feeds from the injectors. The blowdown valve is the Martin Evans design ? using a stainless ball which is supposed to be more reliable than the sort using a tapered valve. The ball is free to rotate when the valve is opened and the theory is that the rush of steam and water through the valve rotates the ball and keeps it clean.

Components of the blowdown valve

The body of the valve is fabricated from bronze bar and the outlet stub is threaded internally to take a piece of copper pipe to discharge the water etc. below the frames. The operating spindle has a cupped shape depression in the end to centralise the ball when the valve is closed and the outer end is retained by a threaded collar which prevents the spindle being unscrewed completely. The end of the spindle has a pin pressed through it so that it can be turned with a slotted bar. This is much easier than the usual practice of filing a square on the end!

Blowdown valve fitted. Also one of the clacks

The above photo of the backhead also shows one of the injector clacks. I actually made this earlier when I did the hydraulic test on the boiler as I found the handpump I used for the test leaked very slightly and the pressure kept dropping slowly. Fitting this clack as well solved the problem. The clack follows the usual simple design using a 3/16" diameter ball but uses a 1/8" bore 'O' ring for the valve seat rather than a plain metal seat. The 'O' ring was reduced in diameter to fit inside the clack body (as done for the blower valve spindle seal) and fits on top of what would normally be the machined seat. Using an 'O' ring gives a perfect seal without the bother of having to cut a metal seat with a 'D' bit and reaming the inlet hole. I think a lot of commercially made clacks now use 'o' ring seals. If the seal does get damaged it's only a moments job to fit a new one.

Backhead clack with 'O' ring seat for the ball

The two clacks for the axle pump and hand pump feeds (at the front of the boiler barrel) will be very similar but I'll try and reduce the physical size of these as they are so prominent. I think I can get away with using 5/32" balls for these which means I can reduce the diameter of the body slightly. I will stick with 3/16" balls for the injector feeds to reduce the obstruction to the water flow as much as possible. In any case, these are at the bottom of the boiler inside the cab so are not so visible.


It seems a long time since the last update but I haven't been slacking! With the boiler fittings nearly finished I was itching to do a proper steam test and just needed to make a safety valve before I could do this. Helen actually has two but I only made the one to begin with. The valve follows normal practice and uses a 3/16" stainless ball on a 1/8" diameter seat. The retaining/adjusting cap is drilled with 6 No. 51 (0.067") holes which give a total area 50% greater than that of the valve seat so there is plenty of room for the steam to escape. The spring was wound from 26swg stainless wire, the size just being a guess for now. I would no doubt have to try different lengths etc. to get the right release pressure.

Safety valve

The boiler was then fitted to the chassis and the steam pipe connected up and the blower jet and pipe fitted in the smokebox. The hand pump that I used for testing the boiler was connected up along with a water supply and we were ready to go! I chose to use the gas torch to fire the boiler by poking the nozzle through the firehole rather than mess about with a proper coal fire (besides, I had not made the grate yet and I didn't have any steam coal anyway!) This was far from ideal as I think most of the heat went straight up the tubes and I don't think much was utilised in the firebox. In any case it took a long time to raise steam as I was careful to only use a soft flame from the torch. It was necessary even with the gas flame to have an artificial draught until sufficient steam pressure was raised for the blower to be used. I used an electric blower made from a 24 volt government surplus blower purchased off Ebay (where else!). There's loads of people selling these now and they just need an adapter making to fit the chimney. They produce more than enough draught on just 12 volts and are very quiet and vibrationless in use. The one snag I can see with them is that the fan and motor are integral in an aluminium housing and the whole thing gets extremely hot! They are extremely well made with ball race bearings (must have cost a fortune to buy new!) but I am a bit dubious as to how long they will stand up to this sort of use. They were intended as cooling fans for electronic equipment and meant to blow cold air, not hot. As it happens, the proper steam blower works extremely well and can be used at about 10psi so the electric one didn't have to be used for too long.

The initial steam tests showed up several problems, the main one being that now the cylinders were being fed proper superheated steam, the valves were still too tight and seized after only a short run. I therefore spent quite a bit of time sorting this out by repeatedly removing them and skimming a few tenths of a thou of at a time until the chassis would run reasonably freely at the maximum pressure ( and temperature) of 90psi. Fortunately this is not a big job and I could do all three valves and refit them in about 15 minutes. As expected, the valve bobbins showed signs of tightness mostly in the centre where they are directly exposed to the incoming steam. I reckoned that any slight remaining tightness would disappear with more running. I don't know what sort of temperature the cylinders are running at but my electronic thermometer reads to 150° C and that disappeared off the scale! I reckon we must be looking at 170-180° C!

Apart from a few steam leaks (mostly from the joints where I had used Foliac) and one cylinder drain valve which doesn't seem to shut off properly, the only other problem was that the lubricator didn't seem to be pumping any oil (or at least very little) as the oil level didn't go down any. Removing the lubricator from the chassis and turning it by hand revealed that no oil at all was coming out of the outlet union so the valves etc. had probably been running dry! That wouldn't have helped matters. I decided that the springs in the check valves must be too strong so I stripped the pump down and fitted weaker ones after which the pump seemed to work ok. However, after putting it back in the chassis and doing another steam test, it appeared that the valves were now not sealing properly and steam was blowing back into the lubricator tank! I think it thought it was a displacement lubricator!

So it came to bits again! This time I tried sucking and blowing through the valves and they definitely were not sealing again once they had lifted from the seats. When I first made the lubricator I had a similar problem and fitted two small 'O' rings in the valves to form the ball seats. This seemed to cure the problem then but it had come back. Running out of ideas fast, I decided to try getting rid of the 'O' rings and going back to metal seats for the balls. I recut the seats, re-reamered the holes and tried again. It seemed to work ok again so I put it back in the chassis and did another steam test. Bingo! It now pumped as it should and the oil level gradually went down. Quite a bit of oil appeared around the top of the chimney liner and the chassis seemed to run a lot smoother. Hopefully that's the problem solved now. Perhaps the original trouble with the lubricator was just a poorly cut valve seat which was leaking slightly?

Full steam ahead!

The one safety valve coped easily with the excess pressure but this may not be the case when firing with coal. The 'guestimated' spring has proved to be spot on as when the release pressure is set to 90psi, the top of the adjusting cap is about flush with the top of the valve casing! It's action is not perfect as it drops the pressure by about 10psi before shutting off again and it sometimes doesn't seal again properly. I'll see how the second one turns out. It's not a true 'pop' valve and has a fairly gentle action but it still makes me jump every time it goes off, even when I'm expecting it!

The handpump proved to be too small (it's only 5/16" bore) and a lot of vigorous pumping was necessary to keep the water above the bottom nut of the gauge glass. The proper one will be 1/2" bore and should cope with emergencies adequately. The two axle pumps and the injectors should have no problem maintaining the water level anyway.

I decided that these initial tests showed that there were no major problems now with the boiler and the chassis so I could carry on with the rest of the construction work.

The next major items were the grate and ashpan and I had been giving much thought to these over the last few weeks. The grate extends over the two rear axles so it would be very difficult to fit the normal sort of grate which just drops out when you remove the ashpan. I eventually decide to make the grate in two sections, the rear long section being fixed and the front sloping section hinged so that it can be dropped down to empty the fire at the end of a run. The ashpan itself would be removable from underneath and have slots in the sides to clear the axles and the round stretcher in between them. I decided to remove the second round stretcher in front of the third axle as it would restrict the dropping of the front grate section and I don't think it is necessary anyway.

To try and keep ash etc out of the rear horns etc. I've fitted a thin brass 'skirt' around the bottom of the firebox which will act as a shield and guide the ash into the bottom of the ashpan (hopefully!) The ashpan proper will fit over the outside of this. This skirt will also hold the grate in place in the bottom of the firebox. It will make removing the actual grate rather awkward but that shouldn't have to be done very often.

The grate is built up from 1/16"x1/4" mild steel. The spacers between the bars are plain 5BA steel nuts as they happened to be the right thickness and much easier than parting off a load of spacers from drilled bar! I would have liked to have used stainless for the bars but this size seems to be unobtainable and I don't fancy sawing it out of a sheet at the moment! The mild steel should last a season I would think, certainly long enough to give Helen a proper track testing. I will probably make a stainless grate when I strip the loco down for painting later.

Brass inner ashpan and built up steel grate

Inner ashpan and grate in position with front hinged section dropped down

The fire door is a simple disc of stainless steel with hinges milled from brass bar. The door is held shut with a lifting latch and a hook screwed to the backhead. Both the hinge plate and the hook are screwed to the backhead with bronze screws into tapped holes.

Fire door with hinges and latch


The last two weeks have been spent 'tin bashing'. I thought it was about time Helen had some clothes on so I've been working on the side tanks and the rear bunker.

All the platework is made from 20swg brass held together with 1/4"x 1/16" brass angle soft soldered and screwed or riveted in place. I decided to make the tank sides and the bunker sides seperately rather than in one long piece as they are only connected over the top of the door by a thin section and it would be a lot easier to just fit this afterwards. Incidently all the angle used had to be milled square again as it is nothing like 90 degrees!

A start was made on the side tanks with the outer sides. These were cut out with the aid of a 'CAD' template stuck to one blank with double sided tape. The bottom edge of the first side was carefully sawn and filed to the template and then tried in place on the running boards to check the fit. Fortunately it was pretty close and only needed slight filing to get a good fit all the way along. It also fitted the other running board quite well so both sides could be made identical and any slight adjustments made after cutting out. To make it easier, both blanks were then tacked together along the edges with soft solder so they could both be shaped at the same time. After shaping the outline, the holes for the windows were drilled and sawn roughly to shape and finished by milling in the mini-mill which gave nice round corners and straight sides. These window openings will have brass beading fitted later.

Brass blanks for tank outers with CAD template

Tank sides after final shaping

Two narrow strips were then cut to form the bottom of the tanks and bent to fit the different levels of the running boards. A rectangular slot was cut in each to clear the tops of the expansion links. This opening will be sealed and made watertight with a cover made from thin brass and soldered in place. Pieces of brass angle were cut and fitted to the edges of the bottoms to take the sides of the tanks. I actually found it easier to clamp the angle in place and solder it before drilling for a few 1/16" copper rivets. The rivets are only really to make sure the angle does not move again when the next soldering operation is carried out and not for mechanical strength as the solder is quite strong enough on its own. The tank bottoms were then clamped and soldered to the outer tank sides. This time I used 10BA brass screws fitted from inside tapped into the sides rather than rivets. This meant that the ends of the screws could simply be filed flush on the outside and are hardly visible. More lengths of angle were then fitted to the sides of the tanks for securing the ends and the tops.

The inner sides of the tanks have a joggle in them as they are wider at the bottom than at the top. This is to try and get as much water capacity as possible. If I had made them the same width all the way up they would have finished up being very narrow due to the large diameter of the boiler. As it happens they will still only hold about 3/4 of a pint each! The inner sides were screwed to the bottom angles with 10BA countersunk brass screws put in from the outside as you can't see them!

Bottom fastened to tank outer and tank inner ready for fitting

I moved onto the rear bunker next and again the sides were cut out as a pair using a CAD template. The bottom, back, and front of the bunker are simple rectangles but the top of the water tank is a bit more complicated as it has a slope on it to give easy access for firing. It also needs to be removable to allow access to the hand pump which is fitted here rather than in one of the side tanks. The top will be screwed to the side angles with 10BA brass screws and sealed with silicon sealer or similar. It needs to be sealed of course because the water level is the same as in the side tanks. The bunker itself is secured to the running boards by four 5BA brass bolts soldered into the bottom of the tank with nuts underneath the running boards.

Rear bunker after soldering together.

Trial fitting of side tank

A trial fitting of the part completed side tanks showed that I will probably have to file the tops off some of the firebox stays to allow the tanks to sit closer to the boiler and allow room for the insulation but that was expected.

The bunker top is a simple rectangle of brass bent to fit with a filler made from a short length of copper tube soldered in at the top. This filler also allows the extension handle to be fitted to the pump mounted directly underneath. The bunker also has two brass pipes screwed and soldered into the front plate at the bottom. These are for the balance pipes connecting the bunker tank to the side tanks. These pipes will be concealed by the cab floor when fitted. I also need to fit a union for the outlet pipe of the hand pump.

Sloping top of bunker tank

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