Thanks Stuart, hopefully it will provide a bit of a modern view on Andrew Smith's original book which described making this engine on just a lathe

The valve was just on dimensions

So it had to be machined very carefully to leave the sides of the valve square. The slot on the top of the valve was already over 4mm wide but the bottom of the slot was tight enough for the nut. Then the hole for the spindle can be drilled. The later drawing show the hole at 4.76mm diameter

the earlier one as 5.5mm, we need the hole to be bigger than the spindle as we want the valve to float like a butterfly.. If it is tight you will either get the fault that the valve will not follow the port face or that it stick too far out

The valve pocket is also tight

and the original drawing show it being cut with a radius. I could see no value in the radius so I went for a normal slot drill cut pocket and squared up on the valve pocket size.
We can now check the fit of the valve in the steam chest and as you can see it is too tight so the inside of the chest needs to be milled out. We can then try the fit on the side of the cylinder to check if the movement of the valve hits the chest at all.
Jo