How I created my Drawings

The drawings of each type are arranged so they can be opened in individual tabs of a tabbed browser and flicked between to look at the changes and differences between them.
They are redrawn from drawings, mostly Great Western diagrams, in Russell's "A Pictorial Record of Great Western Engines" volumes 1 and 2, but are strictly for comparison purposes: they should not be relied on to be accurate. They are my interpretation of drawings made 70 to 120 years ago for a different purpose by men (and women - there were women working in the drawing office) whose training I entirely lack, and whose work I don't altogether understand.
What I do is to scan the drawings from the books and then use a vector graphics program to completely redraw them. Even if the drawings had been reproduced perfectly in the book then when I scan them I probably introduce distortions because I haven't destroyed my book so as to lay each page perfectly flat on the scanner. What I do is to lay out a grid on the screen which matches the measured distances on the drawing, and then juggle the aspect ratio of the sketch to get the closest possible fit. The actual accuracy is going to be variable from sketch to sketch, but its certainly no better than the nearest scale inch and often will be several inches out.

I also have doubts about the accuracy of the original sketches. As I understand it the original drawings are general arrangement ones used for weight diagrams and the like, and were not used for construction or maintenance. Thus I don't think a sometimes hard pressed drawing office would always have drawn them to the limits of accuracy. I sometimes see what seem to me to be obvious errors. For instance to my eyes the Diagram B74 drawing of the 5700 (fig 272, Russell Vol 1) shows a dome that is taller than the chimney, but the diagram B70 of the 8750 (fig 463, Russell Vol 2) shows one that is shorter than the chimney. In both the chimney is dimensioned at the same height and I doubt that the domes were different.
This seems to have significant implications for the fine scale modeller. I do not believe that the locomotive diagram drawings are of great use to you: it seems to me it would be quite unwise to scale dimensions from them. I fear that what you need are the full engineering diagrams made by the drawing office for the factory to use in manufacturing, which I suppose isn't really a surprise.
I must also repeat the advice given by J H Russell in his introduction to the books. He quotes J N Maskeleyne who said in 1935, "If a model or drawing is to be made, be careful to have at hand a photograph showing the particular engine concerned, and of the period in time desired, as every engine seemed to differ in some way." and continues with his own advice "...watch out for the small details. Boiler fittings are always suspect, chimneys in particular, ao always try and make the fittings please the eye and look like the photograph... Also use the drawings with caution... a drawn line does not prove authenticity."

On the other hand I can thoroughly recommend this exercise to new modellers. Redrawing the item you are planning to model with a vector graphics program is an excellent way to gain a much improved understanding of how the item is put together and how the parts relate to each other, and is a quicker exercise than you might think: certainly far faster than doing it with pen and paper as our grandfathers did. Its a continual surprise to me how much you learn from drawing something. Of course you learn even more by making the model, but it can be a bit late by then!
If you can look at these drawings and gain a greater understanding of how these locomotives evolved over the years then I will have achieved my aims with these sketches, but if you use them as the main source for anything beyond the simplest "plastic bashing" representational modelling then I have done you a disfavour by creating them.