G3 Made Simple
Locomotive Project No. 1
A Live Steam 0-4-4 Tank
A step-by-step construction guide provided
by our contributor "Ginger".
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Part 3 : Design Considerations
Part 4
Part 2
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The full size G3 scale design drawing of James illustrated below is based on the General Arrangement drawing of the prototype, obtained from the NRM Copy Service referred to in Part 2. It just so happens that the A2 size G.A. drawing for the SER Q class is just a shade larger than the size of the model.
The G3 drawing is drawn to the scale of 13.5 mm to the foot (1.125 mm = 1 inch).
The key reason for producing such a drawing is that it provides important reference points during the different construction phases. These reference points are:
 Wheelbase dimensions
 Length and shape of the frames
 Front and rear overhang at the frame ends
 Footplate height above rail level
 Boiler centre line and boiler dimensions
 Position of the cylinder block

Translating the General Arrangement drawing into a full size drawing also facilitates some critical design decisions and enables the builder to avoid any costly and time consuming mistakes later on.

Front end conflicts. The problem with the majority of Victorian era inside cylinder designs that did not have a leading wheel or a bogie, is fitting the cylinder block inside the front end. The close up illustration shows the tight clearance beneath the smoke box, the front buffer beam, the space immediately above the front axle and also where the valve rods can conflict with the bottom of the boiler. The models by Carson and Bassete Lowke of the LNWR "Experiment" and "Precursor" avoid this problem because the boiler is much higher set above rail level at 8' 7" compared to the 0-4-4 tanks which tend to be circa 7 foot.
Cylinder. A twin cylinder block (illustrated above) is much higher than the single block because the valves are mounted on the top. This protrudes more into the area below the smoke box, but can still just fit into this confined space for most designs (a very tight squeeze for some). Alternatively the block can be turned upside down with the valves underneath. Adhering to full size practice by placing the valves and steam chest between the cylinder bores resolves all of these issues, but that is another story.

Boiler. Although the full size boiler shell measures 4' 4" the dimension outside clothing (lagging) is 4'7". Thus the model will need a copper boiler with a diameter of 2 ¼ inches and allows generous room for lagging. Alternatively one could cheat a bit and make the boiler a bit bigger at 2 ½ inches in diameter. The length of the boiler is 8 ¾ inches with a firebox of at least 3 inches, or shorter if you do not want to follow the prototype dimensions (more of this debate in Part 4). The position of the safety valve/filler socket from the back head end of the boiler depends upon the prototype.

Smoke box. The prototype smoke box for both James and Sam box sits on top of the frames. However the front of the smoke box of James features a wing plate and sand box built into the splasher which mask the narrowness of the frames of the model compared to the full size prototype.

Lubricator. The scale drawing reveals that this important item can easily be located in the generous space behind the front buffer beam and next to the cylinder block.

Using a single cylinder. The space below the smoke box can easily accommodate a single cylinder block as the whole unit, including the valve chest is slewed side ways through 90 degrees. The picture below of a similar locomotive shows the position of the single cylinder tight up against the buffer beam, the close proximity of the front axle and in this case the cylinder block sits well inside the smoke box area. The top of the filler plug for the lubricator is also visible.
The common design features for the models of the SER Stirling Q class and the MR Johnson P class are as follows:
 Round top boiler
 Boiler diameter of 2 ¼ inch, based on prototype diameter of 4'0" to 4' 4".
 Little if any rivet detail on the superstructure
 Slip eccentric valve gear
 Gas fired
 Close adherence to external prototype dimensions

The drawing below of the LBSCR Billinton D3 class, arguably the most attractive of all of the 0-4-4 tanks, is reproduced from Noel Maskelyne's book.
Part 4 will review the off-the-shelf components, Part 5 concerns building the frames.