I've hit a problem and been wrestling with it, off and on, for most of this week.
Having completed the chassis using a chassis jig and the conn rods to set up the hornblocks the holes in the conn rods were opened out a trice using a reamer so that the chassis ran down an inclined bit of track as an 0-6-0 without any binding. In order to clear the inside of the slidebars the front crankpin screws have been replaced with 10BA threaded crankpins. (Although not relevant as they currently play no part I've also fettled the slidebars and crosshead to a smooth but not loose sliding fit and will finish them using the Brasso treatment). I fitted the motor/gearbox (Portescap), driving on the rear axle expecting all to be straightforward, and the chassis ran on the rolling road, initially a bit lumpy but over the course of a few minutes slowing down and eventually binding.
It was clear that at the ten past two position the right hand rear crankpin was solid, with no movement. The left hand rear crankpin was solid when it reached the quarter to three position. (Left hand leads at present, but I reckon that should be a right hand lead). I therefore cheated by opening out the rear crankpin hole in each conn rod using a reamer, reassembled and things were no better. I've now repeated this several times and feel I'm approaching the point where I'll have to bush the rear conn rod holes due to excessive slop! (I probably exaggerate, but you'll understand what I mean). Each time a rear conn rod seizes the crankpins on the other five wheels appear to have some freedom, so I've not, so far, opened out the crankpin holes for the front axles any further.
It appears that, by driving the centre wheel set, the problem goes away. Entirely! I've yet to actually move the motor to the centre wheels but I'm pretty confident that everything will run smoothly.
Note that I've built various, though not a huge number, of locos up to and including this, including (in reverse order) a 9F, A3, Adams Radial , Black Five - even a Dukedog with inside valve gear - and several 0-8-0s and 0-6-0s, so I'm not without some experience of making minor adjustments so that things run sweetly. This one, however, is just defeating me at the moment. And for once I believe it's not anything to do with the kit.
Does anyone have any thoughts about the next attack on the problem? It's now driving me nuts. I've currently taken the chassis back to wheels without conn rods, and all is free running. I've yet to remove the motor to check the free running of the chassis again without motor and with conn rods. At that time I may swap the centre wheel set for the rear just to see whether that makes any difference.
And while discussing things chassis-wise, does anyone know a source of threaded crankpins? I can make my own but would prefer to avoid the faff.
Brian