Making a replacement Bowman chimney
The chimneys that came as optional extras with some of the smaller Bowman engines are often missing, and replacements are impossible to find. The following tutorial on how to make your own is kindly provided by Steve_S, who is a member of the "Unofficial Mamod and other Toy Steam Forum". Reproduced here by kind permission.



My Bowman M158 and M140 were bought as a wreck and a collection of bits and pieces respectively (both described in Restorations) and neither of them had a chimney. The chimney originally supplied with both of these engines, and I think with the M167, is quite a complicated structure. The outward appearance is of a 3/4 inch diameter brass tube belled out at the top, with a hole in the side about one third of the way up. On the M158 the exhaust pipe is fed into this hole, in the same way as on many engines. The M140 uses the same chimney, but it's only for show... the exhaust steam emerges straight from the engine block and isn't fed to the chimney. On both engines the chimney fits over the steam dome on top of the boiler, but the dome is much narrower than the chimney diameter. To make the chimney fit, it has an internal brass tube within the bottom part of the main tube... like this:

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The overall height of the chimney is 3.25 inches, and the seam between parts is 1 inch up from the bottom.

It seems an unnecessarily complicated design... why didn't they just make the steam dome bigger so that a simple tube chimney would have fitted? Who knows? But Geoffrey Bowman Jenkins was right about most things, so he must have had a good reason!

Most model shops in the UK stock brass tubing of various smaller sizes, but not as big as 3/4 inch outside diameter (OD). Eventually I managed to get a couple of 1 foot lengths when I went to the National Model Engineering Exhibition in Harrogate recently... a recommended event... lots of interesting stuff and useful traders.

(For the metric young: 1 inch = near enough 25mm, and 12 inches = 1 foot = about 300mm)

So, first job in making one of these is to bell out the end of the main tube to form the top of the chimney. I happened to notice that the sockets in one of my socket sets have more or less the required profile, so, after anealing the top of the brass tube with my gas torch, I hammered a socket into the end of the tube to get the required shape. I also found another bit of metal... no idea what it is, or was... which I used first to start off the shaping.

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Next I cut the brass tube to 2.5 inches in length and put some masking tape around it 2.25 inches from the top as a marker. Using a cutting disc in a Dremel-type tool I cut 8 evenly spaced slots from the end of the tube to the tape marker. Alternatively this could be done with a junior hacksaw.

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Then, using narrow-nose pliers I bent the 8 pieces inward until they touched each other.

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It seemed to me that they weren't bent inwards sufficiently, so I applied the cutting disc again to the 8 existing slots to create more space, then bent them in a bit further, then I filed the roughness from this end of the chimney tube so that the curvature was fairly smooth.

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Next I made the inner tube from some 7/16 inch brass tubing... you can buy this size at model shops. I formed a bell end on the tube by anealing and twisting the narrow-nose pliers in the end. It didn't need much deforming; just enough so that the inner tube would hang from the bottom of the outer tube without falling out.

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I had deliberately left the inner tube a bit longer than it needed to be, say 1.25 inches, so that I could trim it to the right length later. I cleaned the ends of both tubes where they would touch, applied some flux, and tinned those areas with solder. In order to solder the two pieces together I made a jig out of threaded rod, washers, and nuts. I could then thread the bits onto the jig and they would be held in position leaving both hands free.

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Then I applied heat and a bit more solder, and when it had cooled I tidied up the joint with wire wool. The bottom part of the outer tube is the last part to make. This bit will be soldered to the upper part covering the inner tube. To make it sit tightly against the upper part I ground the inside edge a little.

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Then I cut this bottom piece to its correct length of 1 inch, and filed the bottom end of it it to fit the profile of the boiler radius. The M158 boiler radius is smaller than that of the M140, so the two chimneys I've made are slightly different in this respect. At this stage, after a trial fit of bottom piece, I trimmed the length of the inner tube so that it would not protrude beyond the outer section.

After cleaning and tinning the mating surface of the bottom piece, I soldered it onto the upper part.

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As a final adjustment of alignment l located the chimney on the steam dome of the boiler, applied heat to soften the solder joints, then straightened the upper part of the chimney so that it was vertical. During this I protected the engine by covering it with a wet rag.

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The last jobs were to clean the whole thing up with some abrasive webbing... wet or dry paper would be just as good, then drill the hole in the side for the exhaust pipe. The inside should be sprayed red. I found that it was best to stuff a bit of rag in the bottom end before spraying to stop paint running all the way down inside. Otherwise it leaves paint marks on the steam dome and makes it a tight fit.

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