So now to continue this tale of woe (sorry Tommy), I'll follow the valve linkage back into the steam chest and see what there is to see in that locale.
I should add here that with the steam valve linkage fully disconnected from the eccentric, and free from the crankshaft and flywheel, the bind/drag/stiff spot in the rotation is still there, so its source must be elsewhere, which is good information even if not immediately helpful to resolution.
It was with some trepidation that I removed the six nuts from the steam chest studs, as I really didn't know what I would find, having had the air I tried to run the engine on just blow straight through and out the exhaust, suggesting that the D-valve was not seating. Certainly relieved to find that things actually looked reasonably okay in the steam chest. Did note that the top center stud was loose, so I'll be fixing that one with a bit of shim material to tighten it back up. Both the D-valve and port face seem acceptably flat, and only a bit scored, so some judicious lapping should be all they need, but only after a little file work to dress the actual steam ports, which though not too bad, could certainly use a bit of squaring up and deburring.
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I must confess to being very glad to see some actual liquid oil upon opening the steam chest, as up to this point all the lubricant that I'd come across was either gummy or solid varnish, so seeing some that could actually perform the proper function, especially in such a critical place, was encouraging and even a bit reassuring. At the same time I'll have to say that upon opening up the valve linkage packing gland I found no solid substance at all, where there should be a few winds of graphite yarn or a Teflon O-ring or something of the kind, all I did find was the most sticky, jet black, get on everything and impossible to wash off, tar like compound. Alcohol doesn't touch it, Acetone does little better, Gasoline sort of works it with a lot of mechanical action and any soap short of Lava is hopeless! Even GoJo had a tough time with it!!!
A bit of a mystery, this engine has plenty of them, is the two holes in the top flange of the steam chest cover that are on either side of the center stud. They do also appear in the cylinder casting, so I suspect that they were used with pins to properly locate the steam chest cover on the side of the cylinder casting for properly locating and drilling the steam chest studs. Can't really think what other purpose they would have?
One other issue noted was that the end faces of ears and channel that surround the valve linkage rod have a slight taper that might be why the D-valve wouldn't seat for me, as the jam nuts on either side of the D-valve to locate and traverse it may be just a little too close together such that if the D-valve falls away from the port face it could get wedged between the nuts instead of "floating" between them, so perhaps I'll chuck that up in my mill and square those two faces to be parallel, even though they are only tapering by a bit more than .006".
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I'm glad to report that although still showing evidence of a poor build, at least the steam chest seems to be readily redeemable, and not requiring any too much work.