Scaling Down to O Gauge
by R. G. Neff
Railroad Model Craftsman, October 1949
It is entirely natural for the average modeler to want a locomotive built exactly to scale and with an electric driven model there isn't much excuse for building it otherwise. But this desire to use exact prototype dimensions has been, I think, the cause of many failures in attempts to build live steamers, especially in O gauge. About ten years ago, in Ravenna, Ohio, I saw a 1-1/2 inch scale model of Pennsylvania K-4 built by a Mr. Brown, the owner of a small but well equipped machine shop in that city. It was a beautiful job, built from the railroad blue prints of that engine and complete to the last rivet.
Even in 3/4 inch scale practically everything can be copied exactly from the big engines. I don't know too much about 1/2 inch scale, but when we get down to O gauge, we run into some serious problems. Some working parts are concealed and can be made in any way that gives best results. Others are out in the open, and while some of these will work successfully in exact scale, others just can't be divided by 48 and made to work. You can build an exact scale model which can be made to run after a fashion, but I believe most of you would rather build a successful steamer, with reasonable resemblance to a prototype, even though some of the parts are not quite within scale dimensions.
One part that must be made oversize is the cylinder block. Most locomotive pistons are 4 to 4-1/2 inches wide on the face, which scales down to less than 3/32 inch. Obviously on such a narrow piston you can't have a packing groove or a ring and leave enough of a land for a reasonable bearing on the cylinder wall. Pitons should have one land at least 1/16 inch wide, and if carefully fitted, the engine should run fairly well on bare pistons before adding packing or rings. Rings with a face width of 0.035 to 0.040 and thickness of 0.025 to 0.030 are about as small as you can make them successfully and two rings are better than one. Packing grooves should be about 3/32 inch wide to hold enough packing to do any good, so your pistons have to be about double scale width. I make them 7/32 inch wide and so make my cylinders about 1/8 inch longer than scale.
It is also better to make the cylinder bore close to scale size even though it made the O.D. of the block oversize. It won't be enough larger to be noticed by anyone but a very finicky model builder. We can't make our cylinder three times as big as the scale size of most cylinder head studs, which are 1 inch to 1-1/4 inch in diameter. I have seen models built with 3/8 inch bores to keep the block in scale. The power of the model depends entirely on the pressure exerted on the piston and a 1/2 inch piston has nearly double the area of one of 3/8 inch diameter and a 9/16 inch piston has more than double. As the pressure required to develop a given amount of power is in direct ratio to the piston area, models fitted with these larger pistons need only about half as much pressure as those with 3/8 inch pistons, running nicely on 45 to 50 pounds, which is much easier to hold on the head than 80 to 90 pounds.
Another part which it is wise to build somewhat oversize is the boiler. Some builders will form boilers from sheet, making them the exact size they want, but many of us prefer to make them from copper or brass tubing, and are limited to the stock sizes, 1-1/2 inch, 1-3/4 inch or 2 inch O.D. Perhaps you have noticed that even a light engine on the railroad looks massive when you look up at it, but a model, with its boiler made exactly to scale, looks slender from above. For the boilers of most road engines, 1-3/4 inch tubing isn't quite big enough, even when lagged and jacketed, and 2 inch tubing makes them somewhat oversize. I think this is all to the good for three excellent reasons. One is the additional weight of the larger boiler, which makes better adhesion and more power. Another is greater water capacity, for longer runs and more operating time between stops to pump water. In another article I'll discuss the difficulty of making a mechanical or steam water pump work in an O gauge model. Last, but not least, is the fact that the oversize boiler gives the model the massive appearance that the big gal has from the ground level.