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SMJ Smokebox and spark arrestor


The blower is a bent tube that is not fixed to the blast pipe. After a careless cleaning effort I noticed it wasn't in the proper place. The copper tube is not annealed and difficult to reposition. I bent it to get it closer to the blast pipe and then tied it to the blast pipe with a length of copper wire. Talk about bush mechanics. There is also the exhaust pipe from the donkey pump and this ends behind the blast pipe. Now all three pipes are tied together and should stay in position during smokebox cleaning.

In the picture the petticoat pipe looks slightly skewed. Testing the blast nozzle alignment shows it points up the centre of the chimney. I'd say everything is just fine here. The picture also shows how dirty the tubes were. A later cleaning effort worked wonders here.

While I was here I cleaned the chimney and petticoat pipe. Lots of old dirt came off so let's hope the draught is better now. Before doing this I made a plug for the blast nozzle to keep the muck out. Speaking of plugs, I found a rubber plug for the chimney which is good when I block the chimney to extinguish the fire. Better than a rag.

Blast Nozzle

The blast nozzle was 1/4" dia which seems a bit large for such a tiny locomotive. It now has a replacement nozzle that is 3/16" dia and chamfered at the bottom. The draught seems a little better now and the locomotive has a much nicer beat. Perhaps the large nozzle was fitted because the draught was too good.

Tube Cleaning

After the first test run I checked the firetubes and found that all were badly coated and some were totally blocked. The tubes are 3/8" ID but they all looked much smaller. The first cleaning effort used a long spring from a magnetic pick-up tool and the loco did seem to steam better on the next run.

Finally I bought some tube cleaners and also some cotton cleaners for tobacco pipes. Once the tubes were clear enough to take the plain spring I used the intermediate cleaner which is a long spring with a cotton cleaner wrapped around it. Also I used lots of Penetrene and motor oil to soften the muck. Then I used a proper tube cleaner (bronze bristles) and again used lots of oil. For the final pass I kept the brush clean with degreaser and cleaned the tubes to a smooth finish.

A friend supplied an eBay link to a supplier of gun cleaning brushes. I bought some of their .38 caliber brushes. They come with a male UNC 8-32 thread so I made an adaptor that attaches to a long spring rod from a magnetic pick-up tool. Theses brushes are softer than the bronze tube cleaning brushes which do seem a bit aggressive for copper tubes. The flexible rod is required when cleaning tubes that are behind the blast pipe or obscured by other pipes in the smokebox.

Spark Arrestor

The little loco was throwing embers which were burning my clothes and setting fire to passengers. Not really an ideal situation. Burning clean char reduces the problem. After a few complaints I decided a proper solution was needed. Spark arrestors seem to be very rare and I haven't found anybody who uses one.

Stainless steel mesh would be ideal here. As a first test I used some aluminium flyscreen mesh. The loco was working hard on the test day and I burned a variety of coal and char and lots of fines. At the end of the day the smokebox was full of ash and rubbish. The loco steamed well all day and the mesh showed no damage after the day. It is great that the mesh doesn't affect the draught or the steaming ability of the loco.

The serious spark arrestor shown here is just a 25mm diameter cylinder made from 140mm by 95mm of aluminium flyscreen mesh with the seam tied by a piece of 8A copper fuse wire. The top half fits in the chimney and the seam is not tied here so the cylinder can expand and be a tight fit in the chimney. The bottom of the mesh cylinder is open for the blast pipe and has a slit in the side for the blower pipe. Installation is simply a case of pushing it down the chimney and over the blast pipe. The top of the cylinder is in the chimney above the top of the smokebox. Removing the arrestor is difficult because the chimney is always coated with ash and oil and the mesh sticks like glue. Removal while hot is the best method.

After a torturous trial (firing with coal dust and fines) the spark arrestor totally blocked and the loco died. Clearing the arrestor helped but it was impossible to remove the arrestor even when the chimney was hot. Also, the chinmey bore is less than the petticoat bore and the arrestor didn't sit closely against the petticoat bore. The petticoat pipe has a plate on top and fits inside the smokebox and is held in place by the bolts from the chimney. The new approach is to remove the chimney (4 bolts) and insert/remove the arrestor inside the petticoat. This arrestor is slightly larger than before and fits tightly in the bore. The bottom of the chimney pipe fits inside the petticoat so there is no need to seal the chimney to the smokebox.

The picture shows the petticoat held in place by four bolts while the muffler putty is curing. The new arrestor is 110mm high and sits below the pipe top and is held in place by the chimney pipe tail.

The smokebox needed a repaint so I decided to strip the old paint and try some chassis paint from KBS Coatings. Big chunks of the old paint had fallen off but what was left was hard as a rock and neither paint stripper nor caustic soda would touch it. Removal involved lots of heavy scraping. The KBS RustSeal is meant to stick to clean metal but I just couldn't get it to work. After two attempts I gave up and used etch primer and marine enamel instead. If the enamel can't stand the heat (Norglass assures me it will) I'll try Pot Belly Black and overcoat it with gloss enamel.

The petticoat plate isn't a precise fit against the smokebox so I've made a jig to align the chimney and petticoat with the blast pipe. The jig fits in the blast pipe and pushes the petticoat against the smokebox and keeps it in line while the filler sets. The first attempt here uses Devcon epoxy filler between the petticoat plate and the smokebox. I'll use muffler putty under the chimney base if necessary.

The chimney riser is stainless steel and after polishing it I couldn't bear to paint it. So I've left it in its shining glory and cleaned some of the boiler pipework as well. I'll see what people think on the next public running day. I've used a heat gun to keep the boiler at 100°C for 10 hours to cure the enamel paint. Update 2013-05-27: The Devcon putty survived the test run but the paint has shrunk and torn exposing the etch primer.

The arrestor survived a day burning dusty coal but was soft and tired when I removed it the next day. I went to Wire Mesh Industries and bought some stainless steel mesh. The holes are 1.2mm and the wire is 0.4mm. Now I've made and fitted an SS arrestor which should last a long time and be easy to clean with a brass brush. I had to buy 1 sq m of the mesh and have plenty spare so please contact me if you want some. The left photo shows the aluminium arrestor after one day's use and the SS replacement.

Last modified 2013-01-09

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