Thanks to all who replied. Last night I took another look at the 773 and the tang on the left-hand crosshead is there; I'd seen it the first time I looked at it but it was ahead (meaning forward) of the smoke lever, so the as it moved back and forth the smoke lever was just hanging straight down doing nothing. The tang also looked like it had slid forward 1/8 - 3/16" or so on the crosshead shaft (or piston rod on a real locomotive). As I operated the smoke lever manually back and forth with a small screwdriver, when it was as far forward as it would go, it looked like if the tang was behind the lever, it would jam the crosshead not allowing it to travel far enough forward. This was confirmed last night; as I took a closer look at it, it now looked as though the tang was far enough back on the crosshead shaft to fit behind the smoke lever. I tried fitting the crosshead with the tang located behind the smoke lever, operating it by hand before I reconnected the main drive rod to the center drive wheel. It seemed to operate okay, so I reassembled the rods and tried it on the track under power. The loco only moved a short distance forward before the left-hand side rods jammed to the point that the piston shaft with the tang on it was pulled completely out of the crosshead. I disassembled the crosshead and gently tapped the shaft back into place, but something is still definitely wrong. I feel that I'm on the right track now, but now I'm wondering if the smoke lever needs to be "adjusted" (meaning carefully bent forward to allow it to travel farther forward), or maybe there's something limiting the upward travel of the smoke unit piston. I'm going to have to remove the boiler shell from the frame and take a look at the smoke unit itself to check for anything that might be amiss.
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