The parts of a steam engine must be machined, inspected, assembled, and safety tested. It is the inspection, regulatory and insurance costs that adds the most to larger American made models Often you can save money by purchasing a kit, either way, stop a moment and think about this question . . . Remember, in general, you will get what you pay for. If you wish to spend more then you can get an what is in essence an exact, if reduced scale, reproduction of a full sized engine. We have a short film, courtesy of one of our suppliers.
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If you are unable to view the movie, this diagram may help to answer your question. The story goes that the guage was an import from our European heritage, where it was the width for wagons. Sun kinks are caused by the thermal expansion of the metal rain when exposed to the sun. There is no way to prevent this expansion, but there are ways to minimise the effects. Aluminum has an expansion rate of about twice that.