I used a Wiki Article on Vincent Motorcycles.
Hmm, not exactly the most authoritative of sources.
If the design had called for, say, 47.0 degrees, but the machine shop made them 47.5 degrees, there are a couple of ways to create that error. The surfaces for the cylinder bases could be 47.5 degrees apart but verticals through their centers could meet at the crankshaft. Or, they could be 47.5 degrees apart and the verticals could meet above or below the crankshaft (known as desaxe if done deliberately, the most well known motorcycle example being the Moto Morini 3-1/2). Let's assume the former for simplicity of calculation to see what the consequences would be.
A rough measurement from the outside of a Vincent engine, guessing where the centerline of the crankshaft is, has the mounting surfaces at the top of the heads 12" away. If the angle was accurately machined in the crankcases to be 47 degrees the horizontal spacing between the centers at the tops of the heads would be 16.214". If, instead, they were 47.5 degrees the spacings only would be ~1/8" further apart, at 12.368". Since this would have little practical effect during assembly, holding a tolerance better than 1/2-deg. wouldn't necessarily have been an issue. So, independent of whether the design called for 47 or 47.5, it would be interesting to know what an
accurate measurement of the angle reveals.
