Continuing with the cylinder.
Without disturbing the work the four M4 clearance holes for the hold down studs were added.
Now working from the top the two waste areas can be milled away as before and the holes for the oilers added. I counterbored the cylinder hold down holes but unless you are using large washers that is not actually needed.
The three tapped holes can then be counterbored 1mm deep for bosses like the bearing caps.
Next, the back end of the cylinder can have the six cover holes drilled and tapped M2.5, I had to alter their position on the drawing to avoid a clash with the steam passages.
The inlet side is drilled 9mm x 16mm deep and tapped M10x1 followed by drilling and reaming 4mm the rest of the way through, a standard 4mm drill should just reach.
The exhaust side is again tapped M10x1 by 16mm deep then drilled 6mm for a further 16mm before reducing down to a reamed 4mm hole the rest of the way through, finally use a 8mm csk bit to form the valve seating at the bottom of the M10 hole.
The two steam passages can be drilled 3.3mm into the cylinder bore and the outer part tapped M4 for plugs.
The last hole is the M6x0.75 exhaust port which is drilled up until it breaks into the 6mm hole and then counterbored 8mm x 1mm deep.
You could stop at this stage. Or...
...to round the top of the cylinder I turned up a temporary plug so one end could be supported by the rotary table tailstock and gripped the other end with the jaws expanded inside the bore. It was then just a case of taking a cut rotating 4 degrees (one turn of the handle) and then taking another pass.
The sides were done in a similar way, the tailstock in the 4mm hole and, to hold the other end, I made up a locknut so the M10x1 tap could be used as an arbor.
A bit of work with a file will soon blend in the faceted cuts into a nice curve.
Finally, the dummy bosses can be stuck in with a dab of Loctite.
Part one here Part two Part three Part four Part five Part six Part seven Part eight
