[mr0077]

Simon, was that scuffing only on #5 or typical of all bores? It looks exactly like the evidence I found on a motor I built that I forgot to gap the rings on...they were too tight and caused similar damage to all cylinder walls and pistons. In all but one bore the scratches were too light to snag a fingernail on. They cleaned up with an extra 0.005 honing, but it meant replacement pistons, of course...
I don't think that the rocker coming off would be the cause of this damage, but could it have been the final straw if the gap(s) were marginal? Seems that with the rocker off, the cylinder would simply be unable to exhaust on the exhaust stroke, so it would blow back into the intake manifold when the intake valve opened, then suck in a little air/fuel and repeat the cycle, with no appreciable extra loading on the moving parts.