Making Tectonic progress...on the LH side I could remedy some mistakes. Before joining the two layers of connecting rod I opened the hole of the outer one sufficient to clear the Gibson crankpin retainer (which is soldered to the return crank).
The eccentric rod needs a smidgen clearance from the return crank. I had not noticed the generous array of etched washers, some half thickness and ideal for this spot, that come with the kit.
As I did not have to create any extra length to it, the radius rod is riveted to the dieblock. This gives more range of movement than I managed on the RHS, and to get the most "waggle", the rod is in full forward gear. One could argue that whatever the position of the gear it will be unrealistic some or most of the time!
On this side I had not commenced by soldering the droplink to the crosshead which is the Comet suggestion.
INSTALLATION SEQUENCE AND VALVE GEAR
Once the slide bars and cylinder are fixed to the motion bracket the crosshead may be slid in but not if it is attached to the valve gear, nor even if it has a bolt fixed to it.
However after experimentation I have established it is possible to thread the valve gear into position while the slide bars, crosshead, and connecting rod are all fixed in place as per the photo.
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Testing the loco without the valve gear but with connecting rod, crosshead and slide bars affixed enabled me to fully check for gremlins in the running before going on to the final valve gear attachment stage.
So, I have abandoned the instructions and separated the RHS droplink and crosshead. The valve gear is separate and removable. For this it needs to be bolted on. A 16BA screw head in the connecting rod is soldered to the crosshead.
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The droplink does not seem to very readily fit on the crosshead. It is flimsy and easily bent. I attached some metal so that it 'sits' more positively in the crosshead when held down by the nut.
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It is possible, with the droplink separate to the crosshead, to feed the valve gear assembly into position, provided the valve spindle is not too long, and the combination lever is able to swing up to the angle shown in the photo.
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The shape of the valve spindle pinhead (just as a reminder I am freelancing here with a separate valve spindle) and the cosmetic square section must be minutely filed to achieve the required movement. Once in position the assembly will be retained in the dieblock and reverser shaft with pins which will be removable.
A lesson I have learned before is: never assume each side will be precisely the same! As already found, the valve crosshead is in the right place relative to the end of the radius rod unlike the RHS. But the valve spindle is closer to the outside and so it has to hook on to the combination lever the other way round.
On this LHS the reversing mechanism is located. I can't see part 35 of the instructions on the loco and have left it out.
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Having taken the trouble to photograph the NRM Crab I found myself impelled to make the very simple modification of the kit to represent how the reverser works, with a shaft linking the two passing through the boiler cladding, which drops down vertically behind the frames. This means cutting a little out of the model loco frames, which if visible can easily be repaired with a representation of the boiler cladding through which the shaft passes. A single pin arrangement will be able to retain both reversing arms.
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One thing I had thought was that the reverser was too low on this model and on photos of other ones for the gear to be able to be in reverse. However in fact the arm is cranked and when in full reverse the radius rod is in front of the reversing shaft.
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STEPS
After fixing the steps to the lower slidebars it was clear that there is almost no space between step and union link. So the droplink must not be at all bent. There is just about space given that the step is cleaned up and the rivet filed down. Where things may hit each other I file rounded edges to the collision points.
MOTION BRACKET
This bracket is etched to have a gap at the top. Maybe so that it is easier to assemble the Walschaerts gear. But this is not how the prototype is, and a problem is that this gap weakens the assembly.
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Coupled with this, at least as I have assembled it, the bottom of the bracket fouls the front coupling rod, and would do even if there were no sideplay.
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Significant filing of the bracket was needed to clear the coupling rod. I have slightly bent the bracket outwards which closes the gap at the top a little. With the slidebars firmly attached the motion bracket is reasonably robust. Beefing up this bracket was Tim Venton's suggestion back on Page 1 and this is always in mind. A final step will be to fill the gap at the top.