Perhaps the machining on your crank pins or drivers wasn't done as far as it should have been, wouldn't be that far of a stretch. In general, the mechanism for my Pacific was very easy, and I didn't have to really do anything that difficult. The boiler, though, has proven to be much harder. The tender may be even harder - all of the holes have been pre-drilled... In the wrong locations.
Anyway, I wish you luck, hopefully you don't encounter more big issues.
