In the earlier video showing this engine running with steam, there could be heard a knocking noise. Last weekend I did a tear down to identify and rectify the problem. I found 3 separate issues. First the con rod big end was a bit loose, and required some tightening. Then I found that the threaded join between the piston rod and cross head was a bit sloppy, so that was also tightened, then pinned so it will not move again. (see photo). Finally, and of most concern, the 3 bolts holding the cylinder to the bed were loose, allowing the whole cylinder to move slightly. I think that this movement was what was allowing the piston to hit the cylinder cap in use, causing the knocking. I replaced the BA screws with metric 5 cap screws. Much stronger. Much more permanent. And no more knocking.