I have just bought my first model engine kit, a Stuart D10 casting kit with optional reversing gear. I have read the Stuart book (Smith/Pengwern), looked at various YouTube videos of other people building these engines and have just read Harold Hall's article referenced elsewhere in this forum.

As I am new to model engineering I hope I will be forgiven for asking what might seem to be a very basic question, but I cannot find an answer to this in any of the books or videos referred to and I'd like to get it clear in my head before I start this project.

What puzzles me is this: it seems to me that for the engine to run efficiently one of the most vital factors would be the fit of the sliding valve shuttle against the cylinder port face. I assume that the brass shuttle would need to be kept in close contact with the cylinder to produce a 'steam-tight' seal as it moved to and fro otherwise the steam would take the line of least resistance and just escape to atmosphere through the exhaust, and since there is nothing to hold it in contact this can only be achieved by very close tolerances of the various parts, so that the height of the shuttle and nut is a close sliding fit in the available space (thickness of the valve chest, two gaskets and the cover) with the added complication of it being located by its operating rod. Despite this apparent need for high accuracy I cannot find any mention of this anywhere.

I am not intending to run the engine on anything other than compressed air, but want to make the best job that I can of it, so over to you experts: Am I worrying about nothing or have I missed some vital information somewhere?