ozzie46 said:
. . because of the possibility of rusting after running if just iron or steel were used. Is that not a serious concern?
Ron,
It is a concern, but it is also very much a function of how well an engine is "put up" after its run. Maybe a short description of typical model cylinder practice may help here, so in no particular order . . . .
CRS is not a good material for cylinders unless it is sleeved (in the cylinders) and faced (on the valve face.) It has poor rubbing bearing qualities and will rust no matter what. Cast iron on the other hand is the ideal material because of its inherent qualities, most importantly in wearing and self lubrication (by the graphite it contains.) Also, after a properly lubricated cast iron steam cylinder has run for a while it acquires an oily oxide coating on the cylinder walls and becomes somewhat rust-resistant (but not rust-proof.) C.I. on C.I is one of the best bearing combinations there is although most live steamers do switch to bronze for the slide valves. Bronze for cylinders is just beneath C.I. in suitability and bearing qualities but does have the advantage of being rust-free. Bronze on bronze is not nearly as good a bearing combination as C.I. on C.I.
A very large percentage of cast iron cylinders, from about 1.5" and larger, are also sleeved in cast iron, as were all full size locomotives, and the preferred material is centrifugally cast iron although we usually take what we can get. Bronze is most common these days for cylinders under about 1" diam and are usually never sleeved. One could sleeve a steel or cast iron cylinder with bronze but I don't hear of that being done too often. Oddly model stationary engines, such as Stuart Turner and others, are practically never sleeved no matter how large the bore, at least the original designs never call for sleeving.
All piston/cylinder combinations in any material and size will benefit substantially from having rings fitted. For small cylinders, perhaps 3/4" diam and smaller, O-rings have pretty much taken over as the easiest and most effective ring, especially in bronze cylinders. Above 3/4", or certainly 1" diameter, cast iron rings are the most typical material although I have heard of O-rings being used in larger cylinders. Also, there is a difference between iron rings for steam and automotive or IC rings. Steam rings are made to have a very low wall pressure rating while automotive/IC rings are made to have a high wall pressure and one does not suit the other well.
There are no doubt isolated exceptions to these guidelines but this is generally what is done nowadays. I will add to this if I find I've left out anything important.
