"I say this a lot and, I will repeat it here, draw your model in the scale you will build it. This is especially important in the computer age where many of us do computer drawings. It is tempting to draw what is on the railroad drawing and just scale your drawing. So let 's look at what happens when you do this. Suppose there is 1/4" clearance between some of the valve gear parts and you draw it up that way. When you scale your drawing to O Scale, you now have around .005" clearance. How good are your machining skills? Can you actually make that stuff? Suppose your prototype has a dimension of 4-3 /4" somewhere on it. If you divide 4.75 by 48 (our scale) you get .0989". Great, where are you going to get material that thick? On my models, the frames ,are 4" thick. That's .083" in O Scale. You can't get sheet brass in that thickness , but you can get .080" thickness, so that is what I used. The other problem is all the parts of your model are related. Now that I changed the frame thickness , how does that effect other parts of the model? I decided the outside dimension of the frame, from side to side, was what I felt was important, so I made that to the drawing. That decision now affects dimensions of the cylinders, firing deck, pilot beam and valve hangers. As you draw the model, you will be thinking of how things are going to fit together and how you will make then. I would recommend that you get the mechanism and boiler designed to fit before you start building. The rest of the parts are not critical to your model running."
I think he is right on the target with this statement. This is fact of life when building any model. Rarely can we build a model with every dimension true to scale. This applies to structures, rolling stock and even track. The key in a locomotive is that all parts have to interact and function, and not just look nice.