This engine is all based on a Burrell drawing of one of their engines and as such I’m having to make up a lot of the detail. This detail needs to both look right and still give me a working engine at this small scale.
The angle of the boiler to the groundline has now been reduced to 0.5° and I’ve discussed this in a separate post.
Now I’m working on the size and location of the actual engine, so I’m looking at the bore and stroke in relation to the crankshaft throw, vehicle speed and starting to think about how much steam it will consume.
With this comes the fundamental issue of the crankshaft centre to firebox clearance, the centre of the crankshaft was set by scaling it from the original drawing. This is not the most accurate, but it gives me the correct look. This distance as shown on the drawing is 9.5mm and with a piston stroke of 10mm this means I need a crankshaft throw of 5mm and probably a crankpin diameter of 3mm. This gives me 6.5mm from crank centre to outer edge of the crankpin and so just 3mm of clearance in which to fit the conrod bigend.
There are many different connecting rod end designs, but I’m looking at a fairly traditional split bearing end, similar to the centre design in this set of five/
This is tight given that I need to get a bolt within this 3mm.
Hence I need to raise this crankpin centre by approximately 3mm to give me clearance and space to have 4mm of material either side of the crankpin to allow two 8BA bolts to be used in the conrod big end to secure it.