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You have made a lot of good points Nigel.I have checked the electrode on the plug with a magnifying glass and it shows no sign of burning. The mark on the head are fine particles of the piston. It could be the ignition module as you and Bryan say, the only answer I can provide on that is its the original ignition module before I rebuilt the bike.Going back to your previous post I think may provide a possible clue, the piston material as it was a pattern part? Could this be the cause if the material used wasn't up to Honda spec?My biggest question though is where did all the oil dissappear to over the 200 miles when I checked it (when it was just under maximum). It hadn't used any oil in the first 200 miles and it was leaking any or burning any (I followed the o/h for a good while to check if it was burning any and to check if it was running rich (it wasn't). When I stripped the bike after the blow up there was about an egg cup of oil in the sump but a lot of oil type mist in the air filter chamber, I imagine this was as a result of blow by following the piston damage?The good news is the engineer has begun the crankshaft rebuild today. I have the new mains and conrod. He's going to see how much he needs to remove from the cylinder bore and let me know what size piston I need. When he does I shall ask the piston supplier if they can advise of the materials used and if they are the same as a honda piston. If I can find a genuine piston and rings I'll use those but I think they are even rarer then a sandcast CB750!Thanks for you help