Thanks Doc. Just to bring this back around in a giant circle, I was cleaning up my shed and came across a couple of the pistons that came out when I rebuilt the engine. The one on the left is #6 - a bit hard to tell what caused that catastrophic failure because looking at the coatings on the side the piston was pretty fresh when this occurred. You can see bits missing from the piston crown, the major damage is on the other side where the ring lands are completely missing. The other piston I think is #1 - it's got some kind of gummy substance in and around the rings which I suspect is the residue from running ethanol based petrol like E10. I see a few videos on Youtube now with people pouring various kinds of substances into their petrol tanks to free up their rings from sticky carbon. It seems like a relatively new phenomenon (I mean, rings used to wear out, not get gummed up!). You can also see oily carbon build up on the "good" piston which I am pretty sure was caused by the hellacious oil consumption of the motor. Regardless, when I pulled these out I was amazed by just how little piston there really is any more. The piston skirt used to go 360 degrees around the piston, now you get two sections of skirt and fresh air around the sides. The piston coating is interesting too, pretty much every motor made in the last 25 years has moved to this design or something similar with teensy rings compared to before. Most of the pistons had a mark on them where they had hit a valve, but by the time the engine came to me the timing chain had been replaced. The total failure of #6 I think originated with that incident but either the damage wasn't noticed or the previous owner though "they will be ok" which is never the right answer when it comes to pistons.