The Home Machinist!

That's what the steam dome is for.

Get rid of the automatic cocks.

Some people argue against that too. I have no complaints about what I use.

Buna N

I have had home made inj steam valves with orings on the stem and never had that problem. In 25 years of running I had one crack and leak. I replace them every 2 or 3 years.
What kind of oring are you using? I just used the black ones.

Are you actually getting real superheat? I get a small blast after the engine sits for awhile but otherwise it just drys the steam.
That is a smokebox throttle as I remember it. You may have too much smokebox heat for a ball valve. Maybe too much draft.

Taper pins work nicely too. Whatever you do, don't drive a pin into a blind hole. I had to fix that for someone that did and it was a real pain to get it apart.
Fred

try using a garden hose and flood water all over the the water connections from the tender. If you have an air leak this will usually plug the leak with water and make the injector pick up.
Fred

I don't think it matters.

A friend with this same engine had firebox leaks and could not raise steam due to pressure building and spraying on the fire.

One thing to be careful about: Make sure the assembly is made with a known (and suitable) steel alloy. I knew a guy who used some steel from a scrapyard to build a set of cylinders, and the resulting fabrication was completely un-machinable when he finished! It became a doorstop. The second try wor...

VALVE GEAR

Slide Valves - (Ports) dimensions in inches, as prescribed by Henry Greenly

D = diameter of cylinder
L = Piston stroke
W = Width of ports
P = Length of ports
E = Length of exhaust port
B = Thickness of port bar

W = 1/2D + 1/16
P = L/16
B = 5L/64
E = L/8