The instructions for assembling the crossheads to the slide bars and connecting rod are fairly straightforward, but they make no mention of the fact that the crossheads are not symmetrical and that the orientation they are assembled in matters. Basically each crosshead consists of a back and front which are fixed together with a screw and nut capturing the slidebars and coupling rod. Fortunately I had noticed that the insides of the crossheads were not symmetrical; in the photo you can see that on the right of the hole the piece is inset. This is so that when the two halves are held together the coupling rod isn't locked tight and can rotate slightly around the screw. Putting all the parts together is, however, a fiddly business (you really need three hands, but blutack helps), and on one side I had managed to rotate one half of the crosshead through 180 degrees, so the cutout areas no longer aligned. Unfortunately I didn't notice, as there was still some movement in the coupling rod. I finished assembling the motion and connected it to the frame and wheels of the locomotive only for it to lock solid. The problem is that the misalignment caused the crosshead to run slightly out of line along the slidebars, and when the other end of the coupling rod was fixed to the wheel this misalignment was enough for it to lock. Fortunately this was only a test fitting so I hadn't trimmed down any of the screws, although disassembling to correct the problem still involved removing super glue and cleaning the parts.
I still don't have a fully assembled set of motion to show you, as while one side is now properly assembled and running the other side is still in pieces; it turned out that the slidebar brackets was slightly out of shape, which again led to the motion locking. I've now stripped this one back and filed and reshaped the bracket so it should run smoothly once re-assembled, but that will probably have to wait until tomorrow.
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