David,
Darren has got it in one.
Normally, when making small steam engines such as this, the easiest to replace part is made to be sacrificial. In this case the piston.
So really the piston should be made from the material which will wear out first.
In this situation, you are in a dilemma, your cylinder is already made from one of the softest metals, and really, going by the above rule, you should use something like aluminium, which is not ideal because it galls too easily.
I have experimented over the years with solutions to your problems, as I still use brass cylinders sometimes in the engines I make, although normally I use bronze.
Unfortunatley, because your engine is a single acting oscillator, it uses the piston as a basic crosshead, so not only is it going in and out, supplying power on the out stroke, it is also controlling the swing of the cylinder using the piston side walls as the guide. It is that action that causes all the wear.
Because you are using saturated steam at rather low pressure, and also (even though it doesn't feel like it) a lower temperature, opens up two ways of easily getting what is required.
Both of these solutions are now open to a man of your calibre.
The first and easiest would be to make the piston from PTFE. It has a very low coefficient of friction and expansion, and wears very well. I personally would make the piston slightly longer to give the maximum bearing surface on the walls of the cylinder. So that would require a bit of working out to make the piston crown be in it's normal position, but the skirt longer.
The second, and more difficult method would still retain the brass piston, but you would fit an o-ring as a sealing component. This allows for a bit of a sloppy fit due to wear, but still retain the steam tightness. There are two types of rings that can be used, Viton or silicone. Again, I would make slightly longer pistons if possible.
Ringed pistons do take a bit more 'breakout' pressure to start, but once running, they act as normal.
Viton would be the preferred material because of being so robust, but does require a caution due to compounds produced if overheating the material (not normally encountered at the temperatures you are operating at). I use the Viton o-rings in the engines I produce.
When subjected to open flame type temperatures, to put it simply, it produces a very nasty acid that eats away tissue and bone, in a lot of cases, it requires amputation of the affected part to stop the spread to other parts of the body. But don't let that put you off using it, what are a few limbs between friends. Joking apart, it is perfectly safe to use when in the context of what we do, but the warning has to be given.
Silicone, although very good with little or no side effects, is a little less robust and the wear properties are not as good. Contact with WD40 causes them to swell up and become useless in a very short time.
It takes a little bit of working out and fine machining to fit them, but I don't think you would have any trouble with that side.
There is a third way to prevent wear, and that is to transfer the wear area to outside of the cylinder away from the cylinder wall, by the use of an external guide. That would require a bit of fine turning, but again, well within your capabilities. You could also retain your standard brass pistons, as less wear would be encountered. If you want a sketch, just ask. The main problem would be cosmetically, the engine wouldn't look as it came from the factory.
I notice that you don't have an in line lubricator. That is one of the first bits you should fit to help solve the wear problem, and they are dead easy to make, as I showed the members on the Mamod site.
I am sorry to have gone into it rather deeply, but small engines are a bit of a passion of mine, and if you can grasp the basics of operation, they are really very easily 'fixed'.
John
