Edward Adams's 2½" Gauge 'Monstrous'
With the Winson Britannia out of the way it's time to get back to the Monster!
I decided to buy the loco after all so I am happier now about doing work on it and making a few improvements.
I've had a really good look at the boiler and decided that it's going to be too much work to make it right. Apart from the hole through the water tube in the combustion chamber, the firebox crown has bulged in the middle as has the outer firebox wrapper. It looks to me as though the boiler has suffered too much pressure at some point and been over stressed. I doubt that it would pass a modern boiler exam. Also, quite a few of the firebox stays were found to leak when I tried to pressure test it initially and sealing them would be pretty difficult as the inner firebox is thick with carbon and I doubt if I could get the surfaces clean enough to even use Comsol to recaulk the stays.
So, I've decided to scrap the original boiler and build a new one to the same dimensions. I will be keeping the old one so it stays with the loco. However, I will not be fitting a combustion chamber this time but will go up a size in the tubes to help with the increased length that they will have to be. I've ordered the materials and am now £400 worse off. The price of copper etc. is ridiculous nowadays! Fortunately, I won't need to buy any silver solder as I've got a large stash of Silverflo 55 that I acquired some time ago plus some Silverflo 40.
Whilst waiting for the boiler materials to arrive I've stripped the chassis again to put a coat of paint on it and the wheels to keep future rust at bay.
Before doing that though I decided to investigate the slight leak from the exhaust when I last ran the chassis on air. By a process of elimination I pinned it down to the middle cylinder and I think that the valve was not sealing onto the portface very well allowing air to leak underneath it straight to the exhaust port. The outside valves seem to be fine.
After removing the cylinders from the chassis I rubbed the middle cylinder port face on some wet and dry on the surface plate and it did seem low around the ports. After a bit of work the port face now seemes flat and hopefully that will solve the problem. The valve itself was perfectly flat so it wasn't that that was the cause of the leakage.
Whilst I had the cylinders off I decided to drill and tap the middle cylinder for some drain cocks which it didn't have originally. I think it will need them as there is a large mass of metal in the cylinder and there will be a lot of condensation until the cylinders warm up. I'll probably fit some automatic cocks as the chassis does not have any means of manually operating ordinary ones. I'm not sure how those on the outer cylinders were meant to be operated as there was no linkage to them.
Anyway, back to my favourate job of painting (not!) but it will be nothing fancy. I'm just going to give the chassis a coat of etch primer and then use satin black paint. The rest of the loco was originally gloss black and that will stay the same.
I've got some photos of a full size NZR Kb class loco that is on display and it looks as though the whole loco has been painted matt black, motionwork and everything! I'm sure that's not how they were in real life?
Incidently, I'm picking up a part built NZR Kb at the Rugby rally in June so may finish up with another monster to keep this one company! At least the new boiler for Monstrous will be good practice for making one for the Kb. They are both very similar in size and design although Monstrous has a longer firebox.
The materials for the new boiler arrived today from Macc Model Engineering Supplies. They were extremely well packed and it took about half an hour to unpack them! The recycle bin is now full of cardboard and paper!
Not a lot to show for £400. The boiler tube is 18 inches long and the tubes 36 inches. They will be cut in half to make two flues each. I am waiting on a piece of 2 inch diameter phosphor bronze to make the steam dome bush and the safety valve pad. I was toying with the idea of using some SAE660 bronze for the dome bush as I had some left over from making Helen's cylinders but I changed my mind due to the high lead content. I wasn't sure that it would silver solder very well due to the lead so didn't take the chance of it not working out.
I've taken some photos of the original boiler to show the condition of it in case anyone is interested.
You can see the 'upside down' dome bush on the bottom of the boiler which was added to gain access to repair the damaged water tube in the combustion chamber.
The boiler has four 0.625" diameter superheater flues and thirteen 0.375" diameter fire tubes. The new boiler will have four superheater flues again of the same size but fifteen 0.4375" diameter fire tubes so it will have quite a bit more tube area than the original. That will help the breathing a bit.
The backhead is 0.125" copper with the fittings screwed straight in with no bushes. This was quite normal when this boiler was built. Also normal for this time seems to be the practice of putting the bottom fitting for the water gauge way below the firebox crown! If the water gets close to the bottom nut then you are in trouble!
The backhead had a very nice set of butterfly firedoors which will be refitted to the new boiler. This had been fitted with steel screws straight into the backhead and they were all nearly rusted away and sheared off when I tried to remove them. You can see that there is not a lot of the screws left in the threaded holes in the backhead.
This is the bush under the boiler with the damaged water tube in the middle. There is only a very small gap between the bottom of the combustion chamber and the boiler barrel so the circulation would have been very poor. I suspect that the combustion chamber has dropped due to the other problems with the boiler.
A view of the firebox crown. You can't really see the bulge along the middle but you can see where the top of the combustion chamber has sagged where it joins the firebox wrapper. The bulge in the crown is a lot more in the centre.
A better view of the sagging roof of the combustion chamber:
I noticed when taking the photos that the firebox wrapper is only 16swg copper and that may well have caused the problem. I don't think that it was strong enough for such a large firebox, even with the crown stays.
This is the view inside the boiler that I took with the endoscope. The crown stays seem intact and still attached to the firebox crown but I wonder if they have come adrift where they are soldered to the inside of the firebox outer wrapper? I'll have to get the endoscope out again and have a closer look.
Another view of the firebox crown looking towards the backhead. Can you imagine trying to get that clean enough to reseal the leaking stays!
A view of the top of the firebox showing the bulged out sides.
This is the design for the new boiler done in 3D CAD. I've still got a few bits to finalise but should be able to make a start soon. All the boiler plates and the barrel will be 2.5mm thick and well stayed to give a safety factor of at least 10 on the barrel and the plates and stays. The tubes will be 20swg.
I've been very careful to get the new boiler to the same dimensions as the old so it should just fit into place nicely. I doubt if I will be using the original cladding as it's in a bit of a state and has been hacked about to fit the bush on the underneath of the boiler. The joint on the bottom of the barrel cladding had been silver soldered rather than soft soldered so I had to attack it with a cutting disc in the Dremel to remove it which didn't do it a lot of good.
As well as the actual boiler materials I also ordered some 10mm steel plate to make the formers. It's a lot more work making them out of steel but it makes forming the plates a lot easier than faffing around with bits of wood. I suppose I could have used aluminium which would have been easier to work with but the steel was much cheaper!
In the meantime I'm carrying on with painting the chassis and I'll be glad when that's done!
To be continued